Is there a way to make LiveTV recordings delete themselves after say 6 hours, rather than a day, which is the fastest setting in the setup options?

Alternatively, it there a command that I could run that would tell myth to delete all recordings marked as LiveTV?

This topic comes up pretty regularly, but I still don't know why people get so obsessive about it. The system rules are already really good about cleaning up live recordings.

They're also quite aggressive about cleaning up obvious cruft. I was watching a movie yesterday after a bit of channel surfing and watching half of the movie that was on before it, and when I exited, the most recent live TV program watched was the only live recording in the list. That means it had already cleaned up 5-6 very short recordings from my surfing and 50 minutes worth of the previous movie.

If seeing them in the "all recordings" list offends your sense of order, turn that off using the option at:
Utilities/setup -> setup -> tv settings -> playback -> 4th screen

I've already turned off seeing live tv in my recordings, but thanks.

My main reason for looking into this is just that I want to have a better sense of how much space left I really have for my true recordings.

The place I check this the most is the MythWeb Backend Status page, which tells me how much space I've used and how much that I have free. I don't know of any simple way to get disk usage for just live recordings since that distinction is made in the myth database. That would also be helpful if there isn't a way to delete them.