I suspect it would last for quite some time beyond the officially supported window (possibly until the vast majority of those devices are no longer using Google's map data either via upgrade or retirement). Absent Apple or someone else unreasonably taking advantage of the "legacy device loophole", Google doesn't have much incentive (and some disincentive from the potential PR fallout) to shut it down even no matter what the contract allows. I'm sure sometime after that support windows they'll want to roll out something where it's easier to break existing clients than to keep them working, but until then...