Long enough!
Honestly, I have seen a few times at the track I race where they have made one class run their second 5 minute qualifier and then run a 20 minute main 5 minutes later.
I wanted to make sure that I could get 25 minutes of racing without being out of battery.
Hopefully, in an ideal world, I will find that these batteries do last way to long, and I can get a pair of decent 5000mah or 6000mah to save on weight, but since the cells were cheap, I figured I would go overboard first and then, if I can, go smaller, instead of going to small first and not being able to run.