For what my opinion's worth, I think every individual has an optimum distance, beyond which paddling becomes more of a chore to be endured than a pleasure to be enjoyed. There are so many variables that affect that optimum distance for the individual paddler that the question is almost impossible to answer. It reminds me of the question "What is the difference between a duck?" I have paddled 25 miles in a day (tandem) and been knotally tackered! On other days I have covered less than 10 miles and equally knotally tackered! My advice: paddle until you're just about to stop enjoying it, then stop!
Juvanile delinkwit (retired), vaguely faffing around with a pair of pliers.