I agree with redsnail, i go about 4 times a week on average, run and cardio work everytime, and supplimented with light weight routine.
I think if i remember right, the official is to raise your heart beat by twice its norm 3 times a week, for 20 minute periods each time, but also depends on the person. Swimming is good for you because it works all the major muscle groups, without having to kill yourself , so to speak.
Also important to look at your food intake, type, amount etc, becauseit is like everything, nothing is that bad in moderation.
Best advice would be from your Doctor though, or a competent fitness instructor who can advise
Good luck
