I personally feel that school days are too long for all children. Having said that, though:
Both my kids went to (private) all day kindergarten. In our area, all private kindergartens are full day (so they can charge full tuition!). My ds was 6 y.o. in September when he started, and he had a very difficult adjustment to being there all day long. He was very tired even though they did all the core academics in the morning, and just had fun stuff - art, music, science, Spanish, etc. in the afternoons. In a six hour day they had a 30 minute recess in the morning, a 45 minute lunch period, and the last 45 minutes of the day was called "P.E." but was really just the kindergarteners playing on the playground together!
In contrast, my dd started the same kindergarten (same teacher/school) at age 5 (she had just turned 5 a few days before school started). She jumped right in - loved it, never went through an adjustment period, and was not overly tired when she came home.
Conclusion - completely depends upon the child how well they will adjust to all day kindergarten.
My biggest problem with all day kindergarten was that I missed my kids - 6 hours a day was too long for me to be away from them! LOL!