You can do that. It's no problem to play the game, then take a break for a month, and then play some more the month after. Play only this game and no other games for one month, then take a break and play your other games. Then you can play FF14 again the next month.
You get a month free included with the initial purchase. Adding one month to that would give you two months in total. You can get far in the game in two months, if you just stay focused.
The game disc itself costs about 20 bucks or less, depending on which platform you intend to play on. If you want to get it for PC, make sure it is powerful enough by downloading the free benchmarking tool.
I wouldn't recommend getting it for PS3, as the performance is pretty bad there.