+1. 11+ months is typical and easily the best for logistics and guest satisfaction.
I think there's an outside chance VGC might be able to skirt around that, though. If they refurb 2 floors at a time, they'd need to move no more than 30 rooms at a time (assuming full LO splitting), which is a lot less painful compared to any other resort. Even less burden if they only do 1 floor at a time.
Relocating guests from 1BR/2BR villas would be a significant challenge, though. GCH has some 1BR/2BR suite inventory, but the rack rates are high. I think there's a tiny-but-not-zero chance that Disney-owned VDH inventory could be used to bridge that gap. VDH doesn't have many larger villas itself, but it does have just enough 1BR/2BR if they move fast enough. (before anyone says they wouldn't accept a VDH swap, that's fine, but some guests would, especially if there's a mild deal sweetener in addition to any excess points returned...and Disney wouldn't need many to agree to swap)
Grand Villas have no good solution without removing from inventory in advance, though. The largest GCH suites have too high a rack rate and VDH doesn't have the GV inventory to support transfers. Further complicating Grand Villa refurbishment is that they are 2 floor villas, so they might need to close both floors to refurb the GVs and impact a greater number of rooms.
The safe bet is inventory removal 11+ months in advance.