It really depends on how much of an altoholic you are. With just a couple of of characters you sub a month and powerlevel your characters to 50 to get them the most practical benefits. It's important to be subbed when the character rises up in rank to get all promotion perks.
Alternatively you can try to find an old retail copy on a dustlayered shelf somewhere.
That gives you 30 days Gold for free, some exclusive Bonus items and an easy reinstall if you need One.