Minimum, 15.
JR may be older than the rest of them, but as someone pointed out, the keyboardist doesn't have to move around unless they want to. He also has a fairly youthful, energetic vibe about him, so I don't see him slowing down any time soon.
I don't think JMX is as disinterested as some people think. When I saw them live back in August, I was sitting off of his side of the stage, and he seemed really into it. Ultimately, there's really no way for any of us to know, unless someone here knows him personally.
With the way JLB takes care of his voice these days, I think he has a while left with his current range, even factoring in age. Even if he loses a bit of his range, he'll still be able to sing. Look at Geddy Lee; he's lost a lot more of his range than JLB has, and Rush is still releasing new material.
That brings it down to MP and JP, and I can't see either of them wanting to call it quits in the foreseeable future.