|Charging my Ikea rechargeable batteries with RM7 Mr D.I.Y. dumb charger|
For example; using a 100mA charger (check the specs of your charger for this) to charge 900mAH batteries, you will need 9 hours (900mAH/100mA) to fully charge the batteries.
Some people would recommend to charge longer to make sure not to under charge. So the rule of thumb is to use 1.2 as the multiplier. For this case would be 1.2(900mAH/100mA) ~= 11 hours.
My personal preference is to use the 9 hours charge. Even though I am under charging them, at least I don't spoil the batteries by accidentally over charging them.