As a user, I think 1-3 times a day would be fine.
As someone who pays to host a twtxt feed, I don’t mind what you set it to as long as it’s not unreasonably often. It won’t really make a difference to me. As of about 6 months ago, 3 different yarn.social pods each request /twtxt.txt (with an If-Modified-Since header) every 5 minutes, 24 / 7. I think I also had a different twtxt client requesting it every 10 minutes.