Put them out when it's below zero till they freeze, they become really hard at that point.
Seriously tho, put them out for 1/2 hour each day for a few days, then 1 hour, 2, 4, 6 then your good. Takes a few days for them to acclimitize to sunshine, temperature, wind. Don't put them out when the temperature is different between inside and out by more than 7c, do it during mid afternoon when temps are higher.