Stella loves to fetch, she's 16 months now, and she'd go on for as long as I'd let her. She can be really embarassing though, cos when I want to give her a rest she jumps in the air squealing really loudly, asking me to throw for her so it's not much of a rest!
I usually finish off a session like that by throwing small biscuits into the grass for them to find. That way they are still busy but in a different way.
Their exercise has to be sensible for their age and size, and over-exercising can be worse for them than under-exercising them when they are developing.