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.

https://winterfetish.com/products/darlex-catsuit

Absolute best sleepsack for longer sessions as it is truly breathable through the darlex. Add a catheter/leg bag & basically you can physically stay in it as long as you can handle it mentally (I can only go about 4 hours before I start to get claustrophobic).

This is an excellent product.

The ordinary sleepsack with internal sleeves:
https://winterfetish.com/products/darlex-sleepsack

The sleepsack with straitjacket arms:
https://winterfetish.com/products/darlex-sleepsack-with-straitjacket-arms

I have the first one. My record for staying in it, while wearing a diaper, is 12 hours. This included a two-hour nap in the afternoon.

Never tried this version, but I have spent nights in both of these:
https://winterfetish.com/products/darlex-sleepsack and https://winterfetish.com/products/hooded-spandex-sleepsack.

As you cannot sleep on the side, because of the arms, the supporting mattress becomes important. A thin gym-mat made for a very bad night, but with a reasonable mattress, I had a fairly good night in the darlex version. And it is absolutely inescapable.
The hooded spandex version was more of a problem. During the night, when the muscle tonus relaxed, the spandex started to tighten the body in unusual ways and I woke up all the time because the shoulders started to hurt and the hood pulled the jaw into a strange position. Even when measured fairly tight, I eventually succeeded to escape from the spandex version.

Interesting. I've only ever experienced the traditional leather sleepsack (Fetters I think) and during private play, a repurposed winter weight mummy style sleeping bag with cargo straps.

Is darlex much stronger/less stretchy than spandex. I'm assuming it is not as hot as a leather bag?