Yes similiar to the other posters I do 5-10 mins of training a day, depending what we're doing. Training is very tiring for your dog too, 10 mins is equivalent to a walk for them.
I end the session with play, and have a toy they don't have access to for that - so when the lamb comes out they know it's a special play time.
As Stuart says end on a +ve. Signs the session's going on too long are lack of concentration, panting, pacing and generally appearing stressed - that's the dog, not you!