Longest Possible Safe FW Dip?

[rayjay]

Dipping the tank will only be temporary relief for it as the ich is in the tank.
He has an outbreak now due to the stress of being moved to your tank, and possibly due also to trying to establish a territory that others are not willing to give up on.
I would redo the reefscape so that they ALL have to re-establish their territory at the same time, making it easier for it to settle in.
If ich gets to be quite bad you may have to give it a dip just to give it more time to get established, but removing it to dip it is a big stressor in itself.