Depends on the type of battery. If it's a good type (NiMD or Lithium Ion) then just charge it whenever you happen to be near the charger. That way if you happen to be out with only a partial charge and can't get to your charger for a long period, you won't be saying "damn I wish I'd charged this up last time I was home".
If it's NiCad, only charge it when it's fully drained. Don't think any PDA's run on those though.