Looking at his career #s, they're remarkably consistant. He got good in 1995 and has been pretty consistent every year with no drop off whatsoever yet...
How quick a drop off sort of depends on what factors make them so good. A guy who's #s are dependent on some great physical skills, once those skills go, they can drop fast, but a guy who's more of a thinker type player tends to decline slower.
The quick decliners are usually the free swingers who don't walk much. I would say that when Vlad Guerrero loses a bit of his bat speed, he'll decline fast because he has no plate discipline.
Guys like Tony Gwynn or Jeter can play a long time because they're stats come more from their knowing how to hit.
The thing is, Mannny is both a great athlete, and a great thinking hitter, so the drop off will probably be slower and more gradual and even when he's no longer got his great bat speed, he could (if he wants to hang around) have some good years in him, just not with the awesome power he has now. His avg. and. obp. might even go up a bit if he stopped swinging for the fences as much. Odds are though, he's too proud a guy to hang around once he's not one of the top hitters in the league..
But he's got the smarts to be a Chili Davis like DH for a lot more years even after he loses his athleticism if he wants to. He'd still be a tough out.