How often you should be using a rowing machine depends on what you want to accomplish with the rowing machine but if you're trying to lose weight by using a rowing machine you should use the rowing machine everyday for 30 minutes each day in the beginning.
When you use the rowing machine at a moderate pace for 30 minutes per day you can burn off around 300 to 316 calories in those 30 minutes.
If you're looking to just shape your body and build leg muscles then 30 minutes each day every few days or 3 times per week is usually enough.
But of course if you prefer to use the rowing machine more often you can build your muscles faster and lose weight faster the more you use the rowing machine.
I used a rowing machine everyday in the beginning for 30 minutes each day and I lost 100 lbs of weight in 1 year of using the rowing machine and I got a better body shape and stronger leg muscles as a result.