djalam's answer had to do with battery charge duration, so it doesn't really answer the question you were asking.
Most LiIon batteries have a charge life of about 300-500 full-charge cycles before they start to decline precipitously. It varies a bit, but if you're in full "road warrior" mode (you use your laptop primarily on battery power and work until it quits), you'll get about a year to a year-and-a-half or so of daily use out of a battery pack before it just becomes your emergency backup pack. But note that that's only if you're in "road warrior" mode, and most people aren't.
If you mostly run your laptop plugged in, you should remove your battery pack unless you need to charge it. Leaving the battery in a laptop while it's running on mains power kills the battery quickly. There are two big problems with leaving the battery in. One is that all rechargeables have a "charge memory". LiIon batteries are a whole lot better about this than the older NiCd and NiMH technologies were, but they still have it. Over time, the battery will "learn" how you're using it, so if it isn't deep-discharged occasionally, it will eventually lose the ability to deep-discharge. Try not to get into the habit of charging the battery when it doesn't really need to be charged yet, and try to get into the habit of letting it run until you get the low battery warning at least one charge out of three. (If you miss once in a while, it's no big deal, but if "top-up" charges become a habit for you, they'll become a habit for your battery as well.) If you plug your machine in overnight to charge, you might want to consider putting the power supply on a timer so it doesn't get forgotten and "trickle charge" the battery for too long.
The other problem is that heat kills, and most machines run a lot faster and hotter under mains power than they do under battery power, so even if you're charging the battery only when you should, if you're using your laptop while the battery is charging, take the battery out as soon as it's fully charged.
If you treat your battery as the main power source for your computer, it will act like one for quite a long time. If you treat it as a sort of built-in UPS (uninterruptable power supply), then that's all it will be in a surprisingly short time—it'll just give you enough juice to safely shut down, and that's about it. Good battery hygiene is a relatively easy habit to develop, and it makes a huge difference to how long your battery will last. Once you take the time to figure out your usage patterns and plan your battery use and charging around that, you'll get a lot more mileage out of it.
