Stop when it's not fun. Why would there be any other reason? Stopping because people tell you it's lame or something is... well, lame.
Someone upthread said, though, that if you stop playing for good it means you aren't a true fan, and that, I disagree with. I used to love anime in general and some specific ones in particular when I was younger. I don't care about them now, but I LOVED them then. My tastes changed (I won't say I outgrew them, because tastes can change for many reasons) and I don't love them now, but it doesn't make what I felt then erased from my history. It's possible to be a true fan for a period of time, and for the feeling to wear off.
Man, I think it would be tragic if someone kept faking to enjoy something out of some kind of obligation or guilt about it. I can't think of a worse way to ruin something you used to love. And even if I am not an OTAKU OMG of, say, Sailor Moon, any more, I remember it very fondly.
Depending one what it is about Pokemon that a person really loves, they might keep playing forever, or it might not keep their interest anymore.