You really have to listen to your body on that one. I generally don't need much convalescence, and start working out again as soon as I feel up to it (even if it's lighter than I would normally work when fully well). Cardio is usually the last thing I get back into full-force, but I have crappy lungs.

Don't go if you feel you have to fight the whole way through the workout, but as soon as you don't dread the thought of moving, get back to it. Your weight will come back as you get back into the swing...