that's a question like how long is piece of string?
You can start trying to integrate them as soon as you like, but beware that it could be a long and heart-breaking process: especially if the ex-batts are very lacking in feathers.
we tried to integrate ours straight away (we had a week of work when they first arrived) but there was a lot of fighting and pecking.
not just from the originals trying to assert their authority, but from the ex-batts. we even had to separate them from each other! it took about 3 or 4 months for them to be fully integrated, but we did it slowly, where they would liveon their own for the most part then join in the others for playtime around the garden (basically so they could stay away from each other whilst being in the same area)