I think 15 minutes sounds light. I haven't rowed in a very long time, but if you're rowing for that long, can you do something else too? Like going for a walk, stretching, crunches/pushups, weights, running, cycling, yoga, etc? So you could do a 15min one day, a half hour the next, 15min the next, half hour the next, etc. I think it's at least a half hour a day that's recommended, but then again, every time I read a printed recommendation it's slightly different.