In our school, k-garten starts half day, then graduates to full day on Oct 20. My son completed this three years ago, and now my daughter is in, about to make the jump to full day in two weeks. I strongly opposed full day k-garten before my son started school, on the grounds that children are not developmentally ready, and are being asked to do too much in school at that age. For example, art, music, phys. ed. and computer are nice, but IMO are not essential components to the extent that children need to change rooms and teachers for them (these subjects could easily be integrated into the room based regular curriculum). Teachers argue that 5 yr olds need to know more, earlier, and I would argue that our whole educational system is 'pushing down' on our kids younger and younger to know more and more. Also, one of our school's biggest reasons for wanting to offer full day was because of so many working parents, high day care costs, parents not being able to pick kids up half day, etc. IMO, cultural factors should not be a primary dictator of what we do to our children in the name of "education."
That said, when push came to shove, I decided to let my kids try it, because we don't really have other options around here. My son did fine until Oct 20. After full day, he was extremely tired and irritable until Christmas. He eventually did adjust, but it was rough going. And that's my point. If kids have to work so hard to adjust to something, to me (as a child clinician) that means they're not developmentally ready for it. We just worked hard on keeping things quiet and nurturing at home while he was adjusting. My daughter is already very tired when she gets home from half day (8-12:30). I am not sure how she is going to react to full day, though she does want to do it.
I also get to observe dozens of Head Start kids each year and their adjustment to full day k-garten, through my job. It is tough going for so many of them to get used to full day, even when they were very used to a long Head Start program. There are many new demands that come with k-garten, that IMO are much greater than most preschool programs would prepare children for.
So, anyway, those are my thoughts, from a personal, professional and cultural perspective!