Hi AR. Keep mind that Li-Ion cells can be topped of regularly, without any ill effect. Also, the shallower the discharge the easier it is on the cells and the longer they will last (number of equivalent "complete" cycles).
I try to limit the discharge of my LiCo Li-Ion cells to around 3.80 Volts, of course this doesn't always work out, but that is my goal. To more directly answer your question, an open circuit voltage of approximately 3.50 Volts, is for all practical purposes, a "empty" cell. So, that should be your lower limit. Cells can be discharged to lower voltages than that, but should be charged as soon as possible, to avoid damage to the cell. LiCo cells discharged to below a rested voltage of less than 3.00 Volts, have been over discharged. They may still seem to be useable, but likely have been damaged to some degree.
Dave