If I'm guessing I'd say 3-4 years where he can play like he played this year. And then maybe 2-3 after that where you might see another decline. I, like most of us I think, take him at his word that he wants to play as long as he can. I also think his father was probably onto something with the idea that he'll want to play - and want to play in New England - longer than other people think he should, or maybe will allow him to. Even most of the "negatives" anyone could spot in his game are only negatives with regard to himself at a younger age. You need to be able to do everything well to an extent but with BB constructing a roster that is malleable week-to-week in terms of gameplan there's no reason to believe he won't continue to be the best option at QB for several years to come. Plenty of QBs with more mobility, "better" deep balls, and far less mileage on their arm fail to consistently move around the pocket as well, fail to make proper decisions, and fail to make consistenly quality throws in comparison to TB. And that's to say nothing of his leadership and intangible presence.
Despite everything we've seen to the contrary I'd like to think (and do believe) that he is considered a special case by both the Kraft family and BB, and there would be a respectful "out" provided for him should they believe he's no longer the best option to be the quarterback for the Patriots.
So to that end, I'd rather watch him suck for the 2019 Patriots than do anything anywhere else, that I know. The best advice, already mentioned, is probably just to enjoy the ride while it continues to last. Watching his career has been as enjoyable a sports fan experience as I could possibly ask for, knowing there's even more to come is just gravy. Nothing that could happen from this point could convince me that he's not the GOAT. But it's human nature to wonder, and speculate.