Whenever you want, but remember FMLA only covers 12 weeks unpaid time (if you qualify for it and intend to continue working after baby) so taking off too early leaves no time after baby comes. Most women try to or will work until their due dates or a week or so before.
I worked 40+ hour weeks all the way until about 34/35 weeks. Starting around 28 weeks I was feeling worn down and total b I t c h mode all the time. I pushed myself way too.much. I'm now 37 weeks and 3 days and i'm working 15 hours a week and feel much better. I will continue to work short shifts until my baby gets here. I need to keep my job and they are only allowing me 6 weeks off. If they would allow me 12 like most places and we were finacially stable I would take off at 36 weeks.