It all depends on what you hope to accomplish, and what types of activities you enjoy. If you are just a car driving casual visitor, then one or two days is fine. Do you hike? Backpack? Seek great photographic images? Then you need more days. Lot's of great info can be found on-line, including maps and trail guides. Just do it!
I have only been to the park one other time, in summer. I was there with my kids and we spent 3 days in the valley. It was barely enough to see highlights, but be we did hike up to Vernal Falls and played in the pools at the base of Yosemite falls.
Hikes are all day affairs (at least for me) and that was without venturing too far for good photo opps. Park will be crowded that weekend so you may have to venture off the main trails a bit to find that placid setting you seek.
