To keep a Lithium Ion battery good for as long as possible you should keep it charged all the time.
That said, most GPS devices have short battery life. 1-4 hours depending on what its doing, how new the battery is, how often the device is using the speaker, etc etc.
Best solution is too use the car charger when using it. Or at least keeping the car charger in the car and when you get to your destination plug it in (assuming the car keeps the cig lighter on when the ignition is off) then put it back in the windshield when you leave your destination...
The older the device gets and the more you use it the worse the battery life will get. You can replace the battery too, its a little more in depth then replacing the batteries in a tv remote though....