How long a full charge should last depends on the laptop, power consumption and battery capacity. My 1 year old ASUS K24J lasts about 90 mins at normal usage. 45 mins or less if rendering.
If you are asking for "until it needs to be replaced," that depends on when will the battery performance be so low you'll need a new one. Batteries degrade over time even if you don't use it. They degrade faster if you do. As for lap top batteries and how long is the service life of Li-ions, it depends really on how you use it. If you are careless, maybe six months or less. If you take care of it properly, maybe well over a year, or maybe two.