I have a four year old and live in apartments so no priviate yard. However depending on who shes with we do allow some semi unattended play. (a few of us parents that keep check). As for how much. For Oh January- June and from September-January my DD can literly be outside from sun up till sun down or from the time she comes home from preschool (10am) till we call her inside for dinner (around 7pm). Right now though (from late June-August) its just too darn hot she does play and outside but not nearly as long. maybe 10-20 mintues and then shes ready to stop. Summer tempetures can easily exceed 120+ add on the humidity and
:. People die from being outside in this heat too much. The trade off here is we have very mild "winters" days of 60 degrees..
We do our best to beat the heat we try to do outside play early mornings. "water" parkers are big I even get her to an indoor play area which here is like Mcdonalds ect but it still gives her a chance for some big movement. Inside she plays runs around while I'll freely admit TV is used a bit more in the summer by no means are we just sittting around eatting junk food and watching the tube
My kids are often outside playing together unsupervized. If I just had one kiddo I would be less likely to do it that way, but with 3 or 4 of them out there together I feel pretty safe. They have to stay on our block, and can't leave one kiddo out there alone. I check on them, and they check in. I live in a pretty bad neighborhood, but it's all relative I think. It's nothing compared to bad neighborhoods in san diego. LOL