Honestly, it is impossible to know right now. We need to see how much Paul and KG have in the tank this year, and more importantly, we need to see what they do with Ray. If they let Ray walk, and don't replace him with a real, quality player, then I would have to agree that the window is closing. However, I think it is likely that they either resign Ray, or replace him with a comparable player. If that is the case, I think the window will be open for a couple more years.
And of course another huge part of this is also Rondo (and Perk's to a lesser extent) ability to take on bigger roles on the team.