was off 3 months with stress fractures. after 3 weeks completely off to start, did an hour a day of exercise, starting with pool, then stationary bike, then elliptical, missing only a few days along the way. appetite calmed down a bit after a month or so, but despite eating less and 1 hr a day of exercise, i still put on 8 lbs.
into my 4th week back running and the weight is coming off. i initially noticed an increase in weight, despite clearly noticing less fat. now the weight is dropping as well.
i think how much you gain/how quickly you lose is dependent on a lot of factors, such as your age, standard metabolism, training volume, diet. but as a general rule, you'll likely put on a bit of weight, and you'll likely lose it once you start again.
being injured really isn't fun. hang in there.