A pocket/pc PDA is meant to shut down quite some time before the battery goes flat enough to lose the data in the volatile memory.
However, a PDA draws some power (several mA) all the time anyway, so it doesn't take much longer - perhaps 1 more week - before you do lose the lot.
It's a stupid design, done for the business executive lifestyle where it lives in the office or home charger base and travels with the owner for a few days at a time, max. The rest of us are trying to use these toys for real stuff (like flight planning, GPS moving maps, etc) for which it was never intended.