Cordless phone batteries typically last one to three years, depending on several factors such as the model of the phone, the type of battery, and the treatment and usage of the battery. GearFuse says that replacing a cordless phone battery is expensive, so proper care of the battery is important.
There are several useful tips for extending cordless phone battery life. One is to always leave the phone off its cradle until the battery is completely drained. The exception to this is if the battery is lithium-ion. Another tip is to clean the battery contacts on the phone and cradle with isopropyl alcohol. It's also important to keep the battery away from extreme temperatures.