Corss-posted with LWAB:
Basic rule is a short break (~20 minutes) every 50k or so, and longer breaks at meal-times. When I was organizing and someone stopped one of our road captains would ask if they needed help: if they did he would wait then help them catch up, otherwise they would rejoin under their own steam. With the other organizers I know of you're on your own. The UAF also have a rule that if you're 10 minutes late at a halt you're considered to have packed. I was timed out at one stop during PBP 2006 simply because I headed for a loo as soon as I got there and nobody noticed I'd arrived. (I contested that later, successfully.) I don't think that organizers other than the UAF itself apply the rule - I certainly didn't and neither did other UAF Audax clubs I rode with.
UAF-affiliated clubs may also allow unplanned mass halts, e.g. to put rain jackets on, but IME the UAF itself doesn't. I also know a couple of hardened UAF types who won't even sit down & have a coffee at planned stops. I never did work them out.
Nice things: you're always in a pack with lots of shelter, the road captains take the brunt of the wind up front, and traffic calls, lights, etc. are looked after by road captains front & rear, and maybe one or two riding up and down the pack as whippers-in. Meals and sleeping accommodation are laid on. The organizers' aim is to use all the time allowed for the brevet, although a little euphoric sprinting might be tolerated at the end - I did one with the Mulhouse bunch where the entire 70-strong pack was hitting 36 kph over the last 25k just for fun.
Not so nice: it can be hell on the bowels. The choice between "sitting on it" for three hours to the next halt and having to catch a pack doing 22.5 kph after a 10-minute trip into the bushes can be agonizing. And when you get to the planned halt there might be fifty other bods in the queue - or better still, there might not even be a loo available. T'other thing is that delays on the road due to headwinds etc. get taken out of rest time. Again, on a long brevet it can be hard to stay awake after lunch when you're plodding along at 22.5, especially on the 3rd day of a 1000k.
Other UAF rules include no flashing lights and no helmet lights on the road, although helmet lights can be useful during halts.