Google notes that it'll automatically transition users to the new Gmail about eight weeks after the general release. You'll still have an opt-out choice for another four weeks, but after that it's lights out -- you'll switch to the new version with no choice to revert to the old client.

We wouldn't call the schedule a shock, as it was always going to be a question of when Google demanded a migration rather than "if." However, the roadmap makes it apparent that Google doesn't want to waste time or split its user base. You'll be using the new Gmail before the year is out -- it's just a question of how quickly you embrace it.