I put in three, but it's anyone's guess really. What WoW has going for it mostly is that 'everyone is playing it'. What kept me in WoW for so long was that I had a great guild (I still float around their forums and such, just because it's all hilarious), so going to any other MMO would make me leave the good community.
I predict an eventual sort of heat death of WoW. Ultimately, expansions will continue to come out, raising the level cap higher and higher. The level cap has to increase or else everyone just gets bored and gear gets stagnant, you need that total wipe of stuff, not just new content.
Ultimately max level will be so bloody high that raising a new character will become a monumental task (more than it is already, I mean). New players will be repulsed by the concept of getting to level... ninety? One hundred? N? Already Blizzard has put in tricks like linked accounts to offset this a bit, but eventually as the cap gets higher fewer people will come in to play with their friends.
As this happens, older players get tired of seeing "the same content over and over again". Bosses in the first expansion got compared to the vanilla game, doubtless the same is happening between expansion two and the rest. This will cause an outflow of players. It'll be a slow trickle at first, increasing as there's less and less new combinations of tricks developers can come up with.
Reduced intake + increased loss = dead MMO. Give it a couple years and a couple expansions, and WoW is dead.