For Ni-Mah batteries:
Take the battery Mah rating, divide by charger Mah rating, multiply result by 1.15.
For Ni-Cad, multiply the result by 1.2.
1800 mah battery, charger rated at 300 Mah. 1800/300 = 6, 6*1.15 = 6.9 hours, or 6 hours and 54 minutes.
You do the multiplacation because batteries are not 100% effeciant. Ni-Cads only retain about 80% of the power put into them, Ni-Mah a little more.
"Someone in a Prius tried to race me at a stop sign the other day. I couldn't believe it. I had him for the first 100 feet or so but I can only walk so fast."