I *think* it depends on the workout type and whether you have them push the limits. I've tried sending a sim for a jog around the nieghbourhood once every day. It took a while (more than a week, I think) for her to lose the weight but it did go fairly slowly. I'm not 100% certain but I think changing the eating habits makes a difference too; more salads, etc. Not sure how accurate that is though - I've read it in numerous places but I'm not sure if it's true.
So maybe once a day for cardio workouts? Running on the treadmill or something like that? Enough until they get the Good Workout (or whatever it's called) buff, at any rate.