How long can a sub be safely and comfortably kept in a body bag/sleepsack in an extended bondage session? Has anyone tried adding a catheter, so breaks for urination aren't needed?

There's too many variables to answer this question. It would depend on the sub (medications they're on, body condition, etc) the sleepsack, the environment, etc.

How does the environment affect things? Which sleepsacks are better for longer sessions? I'm thinking about trying an extended bondage session, and doing research. Thanks for any help! ????

If you have the ability to cath (either a foley or a good quality external) the sub duration is pretty open ended. Otherwise you a probably looking at a max of 8-12 hours depending on age, urinary tract health etc.

Back in the Max Fisch days there was a sub who played a lot with Amanda Wildefyre. Ms Wildefyre was well known in the scene for doing extended, multi day sessions. I'm not 100% sure but I seem to recall an interview where she referenced sessions that lasted 48-72 hours.

The sub who used the screen name of -h- on MF claimed being held for something like 63 hours in a bondage bag and a similar length of time
on another occasion using a Segufix hospital restraint system. https://www.segufix-germany.com/index_en.php

No idea if it was true but he seemed legit and others on the forum (including Dommes) seemed to accept it.

My personal experience is very much less. I think my max time in isolation bondage in a bag was around 2 hours. Although, it that instance there were things done to increase discomfort so it certainly seemed longer.

When you say "extended" what sort of time frame are you considering? The longer you go the more you have to consider things like hydration, potentially some sort of nutrition (liquid?) and dealing with pressure points developing.

If the room is too hot or dry you may not be able to last as long safety. A sleepsack that is too thick or hard (ones made for suspension etc) or one with a backbone might be too uncomfortable for long term wear but this depends on the sub as well.

Something I suggest is a little cushion between the knees. This person makes one specifically for sleepsacks but you could improvise easily

https://www.etsy.com/listing/1308345662/leg-knee-pad?click_key=d2d2d08baa499580ec8e2f8f3e7bdfe7799f07fc%3A1308345662&click_sum=04caaa16&ref=shop_home_recs_22&frs=1&sca=1

I agree with Domme Lynx--it depends. How far can a person run? (Of course, I am going to make a running metaphor.) If the domme/caretaker really didn't care about the sub, I dunno, weeks? Do you care about basic hygiene? Bedsores?

Realistically, it depends on the sub's capability to maintain the subspace, and everyone will be different. There is this beautiful point in all heavy bondage sessions where you can see the sub's mind float away, knowing this is where he is, and there is nothing he can do to change it. I imagine this is the sensation you are seeking on extending. When those happy endorphins wear off, you will be bored, then restless, and then it becomes actual torture--not the fun, sexy kind. Everyone has a different "tank of endorphins."

I worked with Mistress Avalon and her submissive, who would easily be "stashed away" for 12 hours (with supervision). I have also seen heavy bondage enthusiasts be wholly spent after an hour. Both are fine.