Transmitter battery voltage

[Charger]

Hi everyone, I was just wondering how low I could run the batteries in my radio before starting to have lost range/radio problems.
It's reading 5.0v on the transmitter screen and I think this is probably quite accurate, but as they started at 6.0v, I'm not sure how long left they have, but I still want to get the most value out of them.
I wouldn't want to crash just because of a few AA batteries!