How long does a battery last?

Normal lifespan for a user-replaceable MBP battery is 3-4 years or ~300 cycles, whichever comes first.

On average a battery will last longer if you use it than if you don't use it.
Running on AC all of the time is not recommended.

Lithium-based batteries function best when used fairly frequently but lightly. Apple states "+An ideal use would be a commuter who uses her MacBook Pro on the train, then plugs it in at the office to charge. This keeps the battery juices flowing.+"

Lots more information in this User Tip.

After 3-5 years, whether used or not, a Lithium battery will be pretty much shot. Just sitting on a shelf, they decay from auto-oxidation.

Yes, you need a new battery. Yours is shot.

Be wary of buying them off web sites. If the reseller is just someone who grabbed up old distributor stock that has been sitting on a shelf in a warehouse for 3-5 years, you would end up buying an already dead battery (or one well on it's way to the grave at least).

On ebay, I've always contacted the seller and asked for the date stamp on the merchandise they're selling. If they can't or won't provide it (or if it is ancient, by battery standards), I don't buy it.

For non-unibody MBP's, OWC (www.macsales.com) sells NuPower batteries - I've had good luck with them for my old powerbook, and they are truly new batteries, not old stock.