Hi, where (even generally) do you live, and how cold is it usually at this time of year there? That will probably have the greatest affect on when you get them out. Also how many chickens do you have?
You want to wait until they are all fully feathered for sure, but depending on your Barn/coop set up I would think you can get them out well before last frost. I hatched my first batch ever on October 20th, and they went outside around December 20th. We were still getting some freezing temperatures, but they did just fine.