Maybe you can tell I work at Radioshack?
With rechargable batteries you'll find a variety of performances. Almost any cordless phone you get these days is built to work with around 20 something hours of standby. Even the cheapest ones. That'll usually translate into about a day or two of solid use.
With Nickel Cadmium batteries it's important that you start them out right. You'll want to charge them all the way up when you first get them and then let them drain all the way down as soon as possible. Do that around once a month and you'll preserve the intended battery life.
If the battery life on a cordless phone is only a few minutes or hours than you need a new battery.