Sadly, there's a lack of formal studies to determine how often is too often. I think "pay attention to your body" is generally good advice. If anything is sore or hurting, it's probably time to take a break. Kegel exercises also help keep the sphincter from weakening over time, useful advice even for people who aren't into butt stuff.

It's too much control or trust to place in another person. Even If I didn't have trust issues, It's not something I'd ever be comfortable with. Like you said, to each their own however.