Whatever amount of swimming you do will make a difference. I would think that 2-3 days a week would be good to start out. You don't want to over do it at first because that can just cause problems. I don't know how long you swim for now but you could always increase the amount of time you swim by five minutes or so a day every few weeks. It might not help you lose weight real fast but you will be able to keep the body fat off.