How to decide when to quit

I've been rowing for just over five years now, improved quite a bit, but still pretty mediocre considering how many hours I've spent on the water and the erg. I'm not sure how much longer I can continue training at this intensity and see marginal improvements, while being beat by newer rowers. Ultimately it's my decision to make, but how did others decide when it was time to move one? Any advice would be appreciated!