| As long as you are below the ten hour rate you should not damage your Ni-mh cells at all.
Divide your pack by 10, and if your charge rate is below this figure you can in theory not cook your cells, even if you leave them on continuously. In your case 9000mah/10 = 900mah, so you could in theory charge at 900mah continuously and do no harm, so if you are charging at 500mah then 18 hours (9000/500=18) will give you a full charge.
This rule also applies to nicads, but these should be discharged to 1.1 volts per cell before charging otherwise they will gradually reduce their capacity due to 'memory', ie if you use a 6 volt pack (5 cells), discharge by putting something like a 12v car bulb across the contacts until your meter reads 5.5volts, then start to charge. Do NOT fully discharge nicads, they can go into what is called reverse polarity, another name is bin fodder.
In other words, charge away as you are. | 
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