Well, it's really what you play the games for, that judge how you spend your time playing.
I've been playing WoW for a long time. I've experienced a lot, and had fun experiencing it all. These days I still play it, but about all my time in WoW is raiding with friends. I still enjoy playing WoW, and not about to drop it altogether, but we don't raid incredibly often, and a nice game to fill the time I spend playing games, but not WoW, is really what I'm looking for.
Guild Wars 2. It's B2P model allows me stop playing for large amounts of time, without the thought of losing money, or dealing with subscriptions. Anet's plan of "Making leveling the enjoyable experience, not just endgame" means I can take my time and really have fun, not needing to grind to the level cap to have fun. Finally, it's revamped style of leveling, what with the personal storyline, Dynamic Events, and WvWvW combat, will give me more than it's worth in hours of gameplay. I don't see GW2 as a threat to WoW, not because it's not as good, but because I don't HAVE to choose, I can play both!
Think of WoW as your wife, and GW2 as that stripper model your wife wants threesomes with. Both. At the same time.