We don't actually know how long Thancred was lost in the Lifestream, but given that we don't see him the first time we go through the Forelands and kill Ravana, I think it's safe to say he wasn't out for very long before we meet up again. He tells us that he followed the stories of a primal knowing the WoL would show up, so there's no reason to assume he just took a time out on the first one. Of course, this is getting into the whole "time bubble" issue the game has as a whole where we're still perpetually five years or so after the Calamity despite how much time has actually passed in the real world....