When do you quit flying?
FWIW - Battery manufacturers state the cell's capacity based upon discharge to 0.9V.
On an 8-cell Tx pack, that would be 7.2V. THEORETICALLY, it might be safe up until then. I on the other hand, will stop when I reach 1.1V per cell to provide a safety margin (and many experts agree at this cut-off). This means, with an 8-cell Tx pack, I'll stop when it reaches 8.8V.
I keep stating voltage on a per-cell basis, because I'v heard of some Tx's using 6-cell packs.