I have only done it 3 times but I have had success sending them outside full time at 2 weeks old.
They are in for a week under a lamp, then if it is warm, outside for a week during the day and in at night but no lamp.
Then after at 2 weeks I have put a fish tank with sawdust inside, covered in a towel inside the sleeping compartment of a rabbit hutch. It stays warm in there as not in contact with the cold outside, it is insulated and they are on a thick bed of sawdust.
Means I have had them out full time at 2 weeks old. Only this time of year though as it is staying about 14 degrees ish at night.
Good luck!