Most good quality chargers will turn off, or have a beep when or something when your battery is done charging. If you are using a simple wall charger that has no detection then you will just have to do the math yourself. Check the charing rate on the wallcharger and divide it into your mah of your battery.
For example if your charger chargers at 300mah an hour, then a battery of 900mah would be done in 3 hours.
"Its only a little bit on fire"