Both types of DCS batteries have a shelf life, when not in use, of approximately 1 year. That means it is possible for a battery to sit, without use, for 1 year and still be rechargeable as long as the battery voltage does not drop 2.3 volts.
Battery life is primarily a factor of usage.
Generally, the more a battery is used, the longer it lasts. Improper battery charging, heavy use of auto-focus, frequent card replacements, excessive LCD viewing etc. A NiCad DCS battery will last about 200-300 shots per charge depending on how the camera and battery are used. NiMh batteries will last about 800 plus shots per charge, keeping in mind the same usage guidelines. Typically 1100ma hours.