At this time of the year when people start getting scared about the cold, you might stay as long as the next person allows you to stay an even share the wave if the other one does not mind.
With time we have realized that after a minute or so you get already pretty tired and it's time for the next one to come. Personally, around 2 years ago I managed to surf the same wave (on and off) for 47min, and I had to bail cause somebody else showed up and it was time to share the love.
The paddle back is not as bad as you would think, takes less than 7mins to paddle back to shore through the current.
The only downside of fresh water is that it freezes and that's the reason why we stop surfing the river between late December and mid May...we don't want to ding the boards with the icebergs floating downstream!;)