The only official comment we ever saw was that they had ideas for "twelve to fifteen" expansions, IIRC (basically in response to a question about "well, what's left after Outland?")
In terms of actual, concrete business plans, the leaked schedule had two more expansions listed on it, so I think you can feel confident it'll last for at least that long.
MMO sequels are always huge disasters. People don't want to give up the ages of effort they've put into their old characters, so most of 'em just don't bother to switch.
Plus, I mean, if there were a WoW 2 it'd still just be set in the same Azeroth. It's not like FFXIV or something where they can carry over the overall feel of the game while setting it in a brand-new world.
Honestly, rather than a real WoW 2, I think they'll just expand on the Cataclysm model and revamp the game again.