I'm not a seasoned pro, but to the best of my knowledge, all discharging, especially when done in series should have a cutoff at 0.9 volts times the number of cells in series. Overdischarging cells can be bad for them. So you need to stop the discharge in series when it hits 3.6 volts. There is no way to judge the time, you have to monitor voltage.
Are you saying your pack fully charged in 15 minutes? Any chance it false peaked?