Hello:
We are a battery and charger manufacturer.
For your question.
First: correct one of your mistakes. There is no such thing as "7.2 volts 500 mA" that you mentioned. It should be "7.2 volts 500 mA"
Second: We need to know the charging current of the charger you are using.
Best: If your charger current is 500mA, the charging time will be around 1 hour.
500MAH/500MA=1 hour