Sorry,
it is
not possible to define voltages valid for all Li-Ion chemistries.
Usually you should target a charge of about 50% of capacity and allow a window from minimum 20 to maximum 80%.
The open loop voltage at 50% of charge depends upon the chemistry of the cell:
Panasonic NCR18650B: 50% at 3,6 to 3,65V
LG ICR18650D1: 50% at 3,80 to 3,85V
With the voltages (3,5V or 3,6V) specified by some posters here you may damage some batteries severely!
LG ICR18650D1 will be depleted completely at an open loop voltage of 3,5V!
Even at 3,6V this battery will sit on less than 5% of the capacity.
Practical hint:
Look at battery comparator from
HKJ
chose your battery and display the curve for 0,2A only. With this curve you can estimate which voltage window will be safe for your battery.