I deflasked them somewhere in the first week of january, now the question is when should I plant them out again into a bigger pot, 15 of them are growing beautifully. They say when they have a good root system, but when is that?
Are you asking when you should repot a compot? So long as the plants aren't running into problems you can usually leave them until the beginning of the growing season, next autumn. At that stage the seedlings should be reshooting from new tubers and the new root system will be small enough to handle without doing to much damage.