How addicting is WoW? : FPS / MMORPG / RTS Discussion

I just downloaded the starter edition. I thought it would be boring at first, but the quests were pulling me back in. I was surprised how fast 3 hours went. I'm willing to get the monthly subscription. However, will it get boring later on, I don't want to have wasted 100$ and then dislike it at the end. And is it possible to get 1 month subscription in stores? Thank you!