I will start off with every child is different but they are very flexible too!
My daughter attends a Mont. School 4 days a week full days and has since she was 3 yrs. old (she is now 4 1/2) She LOVES it! And Thrives there!
The great thing about her school is that it's small and very flexable to the children. If she is ever tired or feeling overloaded by the day she can lay down or take a nap on the mats. (because her school takes children starting at 2 1/2, she can decompress at nap time with the younger children) All the children in her age ranges know that they can take a break if they need to. They all have a pillow and blanket that stays at the school that they bring from home, so it's a bit of home.
I was a bit concerend about the all day thing at such a young age but I'm a single mom and she has to be somewhere while I'm at work. The Mont. school is a great place that really cares about her, not just what she learns. I just can't say enough about how great her school is. But check around, drop in a few times at different times she how the teachers are and ask any of the parents (at pick up time) see how they feel about the place. Her school is like a family, a home away from home.