Divide the charge current into the pack capacity to get hours. Add 40% www.rcbatteryclinic.com Red has a good article on modifying wall warts.
Be aware some wall warts are power supplies meaning they have good voltage regulation, other were design as constant current chargers. You only want to use the charger style as the transformer in it is designed to mimic constant current - poor voltage regulation.

The easiest way to tell the difference - what was it used for? Charging a Black and Decker screwdriver - Yes, or powering a set of satellite speaker for your PC - bad. I know I sound like a goofus but the darn things look the same. Make sure they say DC on it also, using an AC adapter is a really quick way to ruin packs.

Use an amp meter in series with the pack to monitor current levels your looking for something around C/10. Keep checking the temperature od the wall wart and the pack if either get hot you're in trouble.

John