I would say that if you do what you're supposed to do in training them, then 2 weeks is sufficient.
I would also recommend letting 6 out one day and the other 6 the next day. Number one, IF some take off on you, at least ALL of them won't take off. Second, with 6 inside the loft, it's easier to get the 6 that are out to trap back in when they see their 6 loft mates eating. Once you've let the two groups out and got them back in once, then you're good to go.
The first time you let them out, DO NOT startle them in any way. DO NOT force them to leave the loft and/or fly. They will do that on their own in a few days. If they come out of the loft and want to trap back in right away, let them. Third, pray that you don't have a hawk attack.