That's so Zen.
As Enaud said, it depends on the charging rate, and I would also add to that the state of charge of the battery before it goes in the charger.
In theory, if you have a 4200 mAh battery at 50% charge state and a 500 mAh charger, it would take a little over 4 hours to charge. If it were fully discharged, it would take a bit over 8 hours.
With a 1 amp charger, that time is cut in half.