I think probably I shouldn't have, but I usually just waited until I could tell the
vape was not as strong. I think they metered at around 3.5 volts by then. People say don't take them down past 3.7 volts or so, but....
My 18650s are still going and some of them are two years old....
Ideally, change them out earlier rather than later. They'll last longer. If you have plenty, then maybe every 6-7 hours or so.....??
I finally got in the habit of changing the battery out when I got home from work, then use the "new" one all evening. Then I'd put a fresh one in in the morning. If you have several, just figure out a routine and you'll never have to worry about it.