2C as Max Discharge Rate
18650 batteries can safely discharge at 2C discharge current, this means that you can multiply this battery capacity by 2 to get the max discharge rate. Eg. 20Ah battery can be discharged in 30 min with a 40 Amp load.
To calculate of load current value with charge/discharge rate, it can be obtained by: C-Rate (C) = Charge or Discharge Current (A) / Rated Capacity of Battery
Expected available time of the battery on given discharge capacity can be obtained by: Used hour of the battery = Discharge capacity (Ah) / Discharge current (A)