Hi, do you mean with or without a heat source? When they're fully feathered out (no more fuzz) at around 8 weeks old they should be able to keep warm with shelter in reasonable weather.
I still gave mine a heat source in their pen after they were feathered since there was some chilly weather heading into winter. Here's a link to more info on raising them the first couple of months: https://www.backyardchickens.com/LC-first60days.html