Blizzard has announced that the PTR for Season 8 will begin on March 11th, less than a week from now. This includes Class Changes and Lair Boss Reworks. The Public Test Realm will be available for a week, ending on March 18th!
An important note is that character boosts will not be available until a couple of days after the PTR has opened. This is important since leveling speed has been rebalanced, especially from 1-25.