I had this discussion with some friends a couple of weeks ago. I'd like to see how others weigh in. I'll hold off on giving my answers for a bit.
There are three versions of the question: If humans simply disappeared, how long would it take before:
1. The casual observer would find no evidence of us.
2. Evidence of our existence would not likely yet be found by us.
3. Evidence of our existence could not be found with our existing technology.
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:
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:
And of all the technology we have, all that will be needed to discover your pyrite laptop is a shovel :)