I usually just put mine on charge the night before I'm going to fly and they're fine. Using the charger that comes with them you will NOT harm them by leaving them on that long. I have a friend that plugs his in when he gets home from flying and NEVER takes them off charge. I'm not recommending that but just trying to show that they are hard to hurt with a standard slow charger.
One of these, along with the proper leads should be one of the first additions to your field kit. Then you will know the condition of your batteries better.
http://www2.towerhobbies.com/cgi-bin...?&I=LXNK81&P=7