If you are currently overweight and can safely lose 28 lbs, then this is feasible and will likely take between 4 and 6 months. First you'll need to assess how many calories you burn during an interval of exercise on a rowing machine. Most machines will tell you how many calories you've burned during a session. One pound of fat is about 3,500 calories. If you burn 100 calories in 10 minutes of rowing, and you row for 350 minutes a week, then you'll lose 1 pound a week just from rowing. First you should make sure it is healthy for someone of your sex, height, and body type to lose that much weight.