You didn't say when you will be there. We have traveled there in the Winter and the crowds are not to bad so there are no traffic delays. We also get up early to be at the gates when they open. This is the best time to see wild life from our experience. Without crowds, we spent three days in West Yellowstone. This gave us plenty of time to explorer all of Yellowstone.
If you plan on going there at a time when it is crowded, I would add at least one more and maybe two days to be able to see everything. There will be times when you are just stuck in traffic and can't go anywhere. If there happens to be an animal near the road, you can sit for a long time before you move again.