Hi, Jladdy. We're going through the same thing here....been exceptionally warm for weeks. We've seen temps in the 60s, unheard of for this time of year. Last night it was 2 below. <sigh> I put mine out at 5.5 weeks, which was April 1st last year. They were pretty doggone close to fully feathered, (the key factor in deciding when to put them out) with still some down on their little heads, but in all honesty I just couldn't stand the dust one more day! Our coop wasn't even finished when they went out. It snowed right after we moved them out, and we got our last snowfall on June 4th.
I got them ready to put out by turning off the heat lamp in the house during the day. After a day or two of that, I started cracking a window in the daytime, just in the room they were in...not a direct draft on them, but it did cool off the room somewhat. Then I turned the lamp off at night too. Oh, they hollered! "Mom, it's dark! It's dark!" But when I finally put them outside they were used to the idea that nighttime is for sleeping, not running around all night. That process probably took a week or 10 days...I can't recall now.
When I first put them out, I did put a heat lamp out there for them and I put a wireless thermometer in there too, with the receiver in the house next to my bed. Stupid, stupid! I'm so OCD that I was awake more than I was asleep, staring at that darn thing, seeing it go down to 20 degrees. I'd get out of my nice, warm bed, put my boots on with my jammies, and head out to check on them. They were fine. I was freezing! I'm embarrassed to tell you how many times the first night that scene was repeated. They were all huddled in a pile of butts and beaks, and not even under the heat lamp. Nope, they were snuggled in front of the pop door. The next night was a repeat, except that I only checked on them once or twice. Again, same story....not even near the heat lamp. The next day it came out and I've never put it back in. Oh, and I stayed in bed from that night on, too!
I can't tell you when you can put your chicks out. I don't know how old they are, what kinds they are (some deal with cold a little better) or what your coop construction is like as far as adequate ventilation and/or direct drafts. But I will tell you to be patient, wait until they are fully feathered (or mostly) and prepare them ahead of time for what is waiting out there the best way you can. I certainly don't want to tell you to go ahead and put them out, only to have you lose some to the drastic change. Nor do I want to tell you to put a heat lamp out there for them and have your coop become one of the ones that we read about burning down. You know your situation far better than I do and what worked for me might be all wrong for you. I'm sorry, I know that you probably would have preferred a more black and white answer, but all I can tell you is what I did. Your final determination has to be made by you.
This year I'm not even using a lamp in the brooder. I'm using a heating pad in the brooder and the little boogers are thriving with it.