Overnight, in most cases is good. 12 to 14 hours won't hurt. Most wallwart (transformer on the wall) chargers charge at 50 or 60 ma and would be a trickle charge on the average 500mah to 700mah packs.
The other issue is what condition is your battery in and how well does it hold a charge. For that you will need a way to check your battery at the field between flights.
I mostly use Voltwatch II installed on my plane
http://www3.towerhobbies.com/cgi-bin...?&I=LXHDJ2&P=7
Some people like to use expanded scale voltmeters such as:
http://www3.towerhobbies.com/cgi-bin...?&I=LXWDZ5&P=7
Even a low capacity battery in the 500 to 700 mah range, in good condition and starting with a full charge should supply enough power for 4 or 5 average flights on a typical glow trainer before needing a recharge.