How soon will you buy a backup battery?

How soon will you guys buy a backup battery? I got my first for my last phone after almost one year. 

i usually buy my replacement battery after a year and a half. when i start noticing that it won't hold a charge for at least half a day with little to a moderate amount of use, like texting or the occasional phone call, not including browsing reddit, or teksyndicate.

Never because i have a droid razr maxx which does not have a removable battery

You would just have to open up the phone, which will probably void your warranty if it hasn't expired already.

usually takes a year. if you can keep your battery from getting super low then longer, when your battery gets super low it gets damaged and you start to lose capacity

i'm expecting my charge port to fail before my battery, seeing what happened to my old phone.