Full Charging Time = Battery Current Capacity (mAH) / Charging Current (mA)
This time is just an estimation because charging characteristics are non-linear and all batteries take very less time to charge from discharge state to 80% and maximum time from 81% to 100%.
In that case the approxmiate time will be
T = 2.4 AH / 0.85 A
T = 0.82 H
T = 0.82 * 60 Minutes
For perfect charging you should use chargers which works on positive or negative delta peaks of battery voltage and turn tehm off when battery is fully charged. One such charger IC is MAX712 from MAXIM (www.maxim-ic.com)