My 1.5 year old just played with his mega block for 50 minutes straight. Some of the time with me, some of the time without me.
Once he had had enough, he proceeded to put them all back in the bag.
This is the same kid who I usually can't get to sit still for more then 2 minutes at a time.
Ps - if you are wondering what I did with that precious time, the answer is absolutely nothing. It was awesome :)