To add to what Da said.
Take the batteries capacity in mah and divide by the charge rate in ma or Amps to get required charge time.
Examples:
1,000 mah battery charged at 500ma or .5 Amps will take 2 hours to charge
1,500 mah battery charged at 500 ma will take 3 hours to charge.
600 mah battery charged at 50ma (typical wallwart charger amperage) will take 12 hours to charge.
Constantly fast charging a battery will shorten its lifespan, IMO fast charging should only be done sparingly, slow charge overnight, like other have said, is best.