Deb, I don't know what FMLA is, but I think you should plan on the maximum time off, then if things go exceptionally well, your boss will be pleased if you come back early.
At 4 weeks post-op, I was able to start kayaking and biking, but doing very easy and short trips, with icing and rest afterwards. I think an 8 hour work day, or worse yet, a 12 hour shift like I used to work, would have been way too much at that time.