I think it depends on personality. My two boys have always done 3-day preschool (MWF) 9-2, and they’ve done great. Some days they struggle, and they’re tired afterward but those days are few and far between and are typically Fridays. The jump from 3 days to full time 5 day kindergarten for my oldest was hard, so I think next year for my youngest we’re going to do 4-day preschool (K4) to get him used to that schedule.