Really depends on how he's feeling, and there's no certain answer at the moment. As far as I know, the dude is still riding horses at age 85, and doing the convention circuit, though. As long as nothing major changes between now, and then, I think it's totally in the realm of possibility for Shatner to do a few hours of shoot time for a 10-15 minutes of footage. Compared to what he does now, that wouldn't be that bad.I think by 2018-19, he will be pushing ninety. That's likely when the next movie will be shot.
But I have to say I would be for it only if it makes sense to do, and is written well.