That is so sad and unfortunate because you are doing such a wonderful thing by donating. It probably depends on the type of work you do. I am a college student and I donated my kidney 4 days ago and I am planning to go back to school on Monday (6 days post op). My surgeon says that I should be fine, but I may be unconfortable sitting and concentrating for a few weeks. He originally took me off of school for 4 weeks, but since I'm doing pretty well I'll be going back much sooner. It seems the biggest hurdle during recover is the lifting restriction of less than 10 lbs, and managing pain, but based on my letter from my surgeon I can assume that the average recovery time is about 1 month or so.
Sorry, I don't know the answer to the FMLA question. Perhaps someone in HR could answer that for you. Good luck on your surgery.