As nickwap mentioned, the capacity of the pack should be on the batteries themselves. The charger should also be labeled for its rated output. Once you have these numbers, some simple math will tell you how long you need to charge.

For instance, if you have a 1000mAh battery pack, and a 70mA charger, you just need to divide 1000 by 70. (1000/70=14.29) This would mean you'd need to charge 14.29 hours, or just round it up to 15 hours. It's not terribly critical at these low charge rates to nail the time right on the money -- a few hours overcharge at 70mA won't hurt anything. But don't just leave it on there for days -- you'll definately cook the pack.

Also keep in mind that the 15 hour charge stated above assumes the battery is flat when beginning the charge. If all of this worries you about damaging your batteries, invest in a decent peak charger and you'll never have to worry about this again. Peak chargers shut down once the battery has taken all it can, and there's no possibility of damaging your pack. Another side benfit is you can charge at 1C (maybe more, depending on the pack), and your 1000mAh battery could be charged at 1000mA. Charging at 1C would charge the battery in one hour (really handy for those short notice flying trips).

Hope this helps.