Here is an image of the average Yellowstone VEI-8 ash layer of 15-20 cm. The rest of North America would be covered in a range of volcanic ash from 1cm-10cm.

The magma volume would likely approach between 300-400 cubic kilometers.

How long would the ash block out the sun?

Depends on where you live. Dr. Dull told me that as long as the VEI-8 didn't occur at the extreme north or south of the planet the entire planet would likely have to deal with little to no sun for at least 2 years. For the Lake Ilopango eruption (which was a approx 6.9 blast) it eliminated nearly all sunlight in the northern hemisphere for 2 years (a VEI-8 would be significantly worse).

Here is a quote from "Michael the Syrian" re: the weather in 537 C.E.:

“The sun became dark and its darkness lasted for one and a half years... Each day it shone for about four hours and still this light was only a feeble shadow...the fruits did not ripen and the wine tasted like sour grapes.”

Back to your question... it wouldn't take more than a month for the entire planet to be clouded in darkness. North America would be in darkness in less than a few days. Also, a VEI-8 eruption lasts for 3-4 weeks (the last one occurred 74,000 years ago in Indonesia).

How much of the ash would coat the vegetation or even pollute drinking water? Are these the concerns?

Those would be major concerns. All of North American land and water would be destroyed. No agricultural production could continue. No water would be drinkable. Depending on the winds land and water in much of the rest of the world would be negatively effected. In fact, the air would not be breathable. Volcanic ash is composed of small tiny rocks. When you breath that air those rocks turn into cement in your lungs.

If Yellowstone erupted out west, what would be the ecological impact to the east coast?

IMO, all of North America would cease to exist as we know it. People on the east coast could be more easily evacuated (if there was international cooperation). People on the west coast would have little chance of survival. As stated above, the blast would last for weeks. No plane or vehicle could get in to the area to save the people there. They would not be safe in their homes (ash would collapse roofs), no water would be drinkable, and all electricity would be cut off. They would need to be in a bunker with lots of bottled water and canned food and of course... tin can could help as well! (if mk and JakobVirgil work their magic in time)

It is for these reason that I have argued that NASA coordinate an analogous program for supervolcano identification and preparation that they have done for asteroids. Asteroids are a massive concern and we should be putting in even more effort to make sure we know when the next one is coming so that we can prepare and avert disaster. But supervolcanoes happen at a higher frequency than asteroids on geologic time scales. A VEI-8 poses a threat to human civilization (I don't think it would cause extinction). We should be investing a lot of money into preparing for such an event, however unlikely. I'm disappointed that not even The Long Now Foundation has proposed such measures.