I don't think there will be any plans for periodic rotating freezing of islands. If freezing is done, it will be indefinitely to single islands. As Tom said, a criteria for reopening could be a minimum player count.
If you disagree with this original flavour of freezing in favour of some other solution, please voice your opinion. I will say that I doubt that any type of merging can be seriously considered due to the sheer dev effort required.
