This isn't as difficult as you may be expecting :)
The easiest way is to go here and type in your info http://www.csgnetwork.com/batterychgcalc.html
However, the correct formulae (if you wish to work it out yourself) is (battery capacity (battery's mAh rating)/charger output (mA)) x 1.5( for NiMh batteries) = time (in hours).
For example, you have a 1200mAh mini pack with 250mA rated charger, so.....
(1200/250) X 1.5 = 7.2hours
If you have a 150mA charger, this will take 12hours. If you have a 500mA rated charger, your battery will be charged in 3.6 hours. So you can see how the mA (milliamp) rating of the charger will effect the charging time.
PLEASE NOTE: The charging times are calculated on a fully discharged battery pack. Obviously, if your battery is only half discharged, you will be 'topping up' the charge and this will take approximately half the time to fully recharge. You may wish to invest in an auto cut-off charger (we thoroughly recommend one!) which has an intelligent charge detection circuit to cut off the charge current when the battery has reached its charge capacity.
Batteries contain an electrolytic paste which dries out a tiny amount each time you charge it. If you overcharge a battery, it will overheat or 'cook'. Cooking a battery isn't good as you can imagine. The heat will dry out the paste quicker and once it is more than 50% dry, it will no longer hold a charge. SO BE CAREFUL!