When the NA datacenters moved to California before SB they were down for 48hrs worth of maintenance, the remaining centers (EU and JP) were only down for 24 hours so anyone on the NA worlds actually got a free day added to their subscription due to the down time. If the upcoming shuffle require longer than a day then I think it would be 48hrs at most, anything longer than that wouldn't make sense at all seeing as the location of the centres aren't changing.