I think it depends on you, what else is going on your life, and what stage of the curriculum you're up to.
After an hour of circuits I was k'put-I'd have to go and have a lie down. Cross country stuff I could do all day and always found instrument stuff fascinating as well.
One thing I did discover, part of the instructors role is actually to put you under stress. - and to see how you react. I reckon this as important as any other part of airmanship.
Don't discount "down time" spent with your instructor over a brew or two. Absorb his wiseness
