This is fun to think about. I've heard other people talk about plastic in the same way that speeding_snail is here. I guess the trick for plastic is that it needs to be hidden from the sun to stay intact. Plastic doesn't bio degrade, it photo degrades. All that plastic swimming in the ocean, as it swirls around in the sunlight, will basically be dissolved in a matter of decades I would imagine. This is already happening really. Once that plastic in the ocean is small enough to fit in the mouth of a fish, that's where it ends up. Speaking of plastic underground though, Geoff Manaugh has some cool things to say about abstract geology where he posits that plastics will one day be very much a part of our earth's geological layers:

Plastics, for instance, “might behave like some of the long-chain organic molecules in fossil plant twigs and branches, or the collagen in the fossilized skeletons of some marine invertebrates.” A hundred thousand Evian bottles, then, may someday transform by compression into a new quartz: vast and subterranean veins of mineralized plastic. In other words, plastics will, in fact, form a new geological layer upon the earth; plastics will be our future geology. It may take a hundred million years, but it will happen. Future Himalayan adventurers will stumble upon vast belts of plastic, compressed into ribbons between layers of bedrock. Volcanoes of the future will erupt, belching transparent magma – liquid plastic – rolling out in great sheets, boiling everything in their path. Unlucky animals will be entombed there, fossilizing slowly over another million years, till their hardened remains seem to hover inside plastic hillsides, like specimens in a resinous vitrine, an open-air museum. Future Darwins will open their sketchbooks, stunned… Fantastically, given time and the right chemical composition, underground stratigraphies of white plastic will dissolve, forming caves. Blurred and colorless stalactites will hang over subterranean lakes in which blind fish swim, unaware of the milky walls and abstract rock formations hovering all around them.

To answer your questions, I'd say maybe just 300 or so years before the casual observer finds no evidence. Depending where they are of course. I live in a city of 2.5 million, in a building over 100 years old. In this case I think it would be longer. 500?

Considering finding evidence of our existence with our current technology, I think that something traceable to us will be around forever - and be discoverable forever (barring some sort of volcanic age where everything melts). It will all be possible because of the petrification of artifact. In fact, the oldest fossil that we've discovered right now dates the original organism of its image back to 3.4 billions years ago. To discuss what might happen to our current cities this amazing article says it better than I can:

After this unceremonious burial, the petrification of our cities will start. The weight of mud and sediment will crush and distort our former homes and factories. While paper and other decomposable materials will be food for the fishes, glass, bricks and concrete have better odds of survival. Most metals will dissolve though, leaving nothing but the plastic shells of all the electronic gadgetry that we have produced. Perhaps some iron interiors will turn into pyrite. Just think of those alien palaeontologists that will be admiring your pyrite laptop interior in 100 million years!

And of all the technology we have, all that will be needed to discover your pyrite laptop is a shovel :)

1. - Thousands.... probably tens of thousands of years.

2 - even longer - 50,000 years at least.

3 - at this point? never. I think we've made out mark.

My dad's a retired archaeologist, and he's coming through town this week. I'll ask him what he thinks about your questions.

This is an interesting one. The casual observer probably wouldn't have much time. I give it about 200 to 300 years before most large traces of our civilisations are gone. Buildings, man made structures etcetera. Like old civilisations, our traces wouldn't be more than small ruins. Might be more, but I think that the casual observer would have a hard time deriving our existence after about 300 years.

The second and third questions are a bit more interesting. We as humans have a huge impact on our surroundings. I mean, we domesticated quite a lot of animals and plants. We build houses, we create art and we even use the power of nature itself to do things which our forefathers would have deemed magic. In our inventive years, we have created one material which has changed our way of living. It is cheap and really easy to use and extremely durable, nigh indestructible even. You might have guessed it. Plastic.

Plastic is one of the most influential materials we as humans have ever thought of. It enabled us to cheaply create strong and durable goods. Things like plastic bags or your garden furniture. It also changed the way we think about design. More and more became and still becomes possible because of plastic.

Now, we were talking about traces of us humans if we were to just vanish. I think that the primary thing that would be left if we were to vanish is plastic (in case it wasn't clear). It is so durable that it is most likely that some of it will never be gone. It might be that it will show up in earth layers that geologists watch at when determining the earth's history. However, when that is not the case, we still have the Atlantic garbage patch and it's 2 cousins.

This is not everything tough. Geologists have already discussed the possibility of a new geological epoch because of human activities. I found a short article here about this discussion.

So here's my answer:

1. In most populated places, about 1000 years. However, I think that many brick, stone and concrete structures would leave visible remnants for even longer, especially where it is dry. Looking at the pyramids, Stonehenge, or the Great Wall of China, it could take several thousands of years with very large structures. NYC would take a long time to revert into unrecognizable rubble, probably 7000-8000 years. It would take a major earthquake or two or some other huge climate impact to get the process really going.

2 & 3. IMO these are probably almost the same. We find plenty of 500 MYO fossils. I have one of a trilobite, actually. Therefore, I think it would take almost a complete cycling of the Earth's crust to completely remove any traces of our existence. The oldest rocks are around 4 BYO, and the oldest fossils are about 3.5 BYO, so I think it would be close to 4 billion years. Basically, everything we leave would have to be subverted and melted.

Of course, as sounds_sound mentions, some of our satellites, the Mars rovers, the moon landing, and the Voyager probes will last even longer. However, they would not qualify for question 2.

The human build structures that are supposed to last the longest are the pyramids at Giza. In their 4000+ years that they are there the desert winds have hardly touched them. Most erosion came from mining the stones to build Caïro. It's incredible how fast stone structures can decay though. A few years ago I walked the Colorado Trail and found several almost dissappeared villages in the mountains that were abandoned less than a hundred years ago. The only remains were rusty boilers, cooking pots and shattered window glass. Nature can be very resilient. Just look at Tsjernobyl.

I believe there was a page on wikipedia where they listed how long what would remain after human extinction but I can't seem to find it. Instead, a link for you: http://discovermagazine.com/2005/feb/earth-without-people/ar...