There is quite a variation between brands and models when it comes to how the wall wart chargers are set up. If you have a way to measure the current when the battery is being charged and know the capacity of the battery, you can make the following calculations. If the current is close to 10% of the capacity of the battery (example, a 600 mah battery and a charger providing 60 milliamperes during charge) you can safely leave the charger connected for 16 to 24 hours. As long as the charge current is about 10% of the battery capacity, this rule holds for Nickel based batteries (not true for LiPo or Pb based cells), an overcharge of at least 40 to 50% is desirable as the charging cycle on Nixx batteries is not very efficient and, at these low rates, will not damage the cells by overcharging them. Charging at higher rates can cause damage, especially if the peak detection circuitry fails. Heat is very hard on a battery so, if at the end of charge, you let the battery temperature rise much above ambient, you are sucking some life from the cells. The hotter they get, the more damage is done. A good source of the care of all types of batteries can be found at the following sources:
http://www.rcbatteryclinic.com
http://www.whenshtf.com/showthread.php?t=2153
http://www.srbatteries.com/nimh.htm
http://dansdata.com/gz011.htm
http://users.frii.com/dlc/battery.htm