I disagree with purple rain's statement, even if it has some published backing. I've seen what keeping your kids home until the second they go into public schools does. If you have not raised them to be considerate they can go in being total d-bags and having to adopt a behavioral pattern after one is established may be far worse of a nightmare than an earlier introduction via a pre-school. This depends on the structure of a preschool as well. Some are play oriented and handle guidance well. Some might as well be a prison court yard where anything goes.
A lot of what preschools do is develop patterns of time. Typically a preschool is not an all day program. At first it may be a couple hours a day, a couple days a week. It may have a nap time. After a couple years, which is generous, your child may not lose their feces every time you walk out of a room. If nothing else, preschool may ween them out of separation anxiety in a way where you're not violating a truancy law if you cave in and take them home.
Both my girls went to preschool. Both did exceptionally well there, and had plenty of kids to play with, activities that we didn't really have at home, and the freedom to play without us hovering over them. They got used to us not being there all day. And they got used to trusting that we would show up later to get them. The patterns of a routine that is different than the one they knew while developing at an age where they can be distracted or in general may be more receptive to change.
Our preschool was 2 years. Year one was 3 days a week. Year 2 is 4 days. Both years a "day" was defined as 3 1/2 hours. 8:30 to 12 noon. I happened to have a job that allowed me to adopt that schedule. I felt any more than this would have been too much preschool.
If it helps to know, both of my girls were born at a time of year that makes them unable to register for kindergarten until they are 6. Both of them took the early entrance exam, and both were accepted into kinder at 4 years old. Both turned 5 within a month or so of entrance so don't get too excited there. Point is, the entrance exam was not an aptitude test. It was a maturity evaluation and a general verification that they can walk in a single file line, take turns, raise hands, share, count to a certain degree, identify shapes, etc. Very basic things. All of which they learned at their preschools.
I don't have a control study here. I don't have an identical family with identical situations choosing the non preschool path, so I can't say if preschool is the deciding factor in why my kids did so well, and are doing so well now. Could be, but could also just be coincidence.
Their cousin... they can't afford preschool. That girl is NOT ready for public school. Just saying...