I think as long as they seem to be comfy and you have them protected from the wind you can leave them out as long as you'd like. I started mine when they were 6 days old and the temp was 70 outside. I placed them in the sun, made a shady spot available to them and blocked the wind by taping newspaper around the outside of the baby run I had them in. I made sure and put out food and water for them too. Their very first outing was less than an hour, but after that I have been letting them stay out from early afternoon until the sun is setting and it begins to cool off. Be warned though, they get spoiled to being out and when the weather is not so nice and they can't go out and play they tend to get rowdy in the brooder!