Colleen, I went back to work, full time, at 7 weeks (not by choice) in my case the first 2 weeks it was very difficult, my knee was very achy and swollen by the time I got home, and I iced and elevated at work, by week 10 I felt much better I didn't even have to ice at work, but the energy level was still not there, I work a 4 day work week and by Friday I am exhausted. I really think that 12 weeks is the magic number, if you can stay out that long you should really consider it.
I work for a very small company, I do not have short term disability or FMLA available to me, you do have that so take advantage of it and make sure you are up to the task of going back to work.