I'm pretty sure something resembling a rotation schedule is posted near the beginning of this thread, but here's a general idea of how it works:
When the first set of the next block comes out (that is, three sets from now, not counting Foundation Sets), all of Rise block will rotate out--basically everything pre-Order. So there's quite a bit of time, although there's no set in stone timetable of dates.
You are correct that old sets will not be purchasable after rotation, but I wouldn't panic yet. Like I said, there's plenty of time.
But yes, there is something approximating a schedule and there will be plenty of advance warning.