An hour and a half, max two hours, we took a mare from Newcastle on Tyne to Southampton, took nine hours and we stopped five times, the last stop was to meet up with my daughter to find her livery yard, each time checked on mare, she was fine the whole way, eating her haynet, but refused water, came off the trailer cool and calm, even though it took two n half hours to persuade her to load
Depends on how long the total journey is and what the horse will be doing at the end of it but normally 2-3 hours then I have a break but if its long distance and mostly motorway then I will go for longer.
I've done 7 hours from Yorkshire to Hickstead with a pony in a trailer. We did do a very short stop to offer water (which was refused) but he was also an extremely good traveller. It was summer so I made sure the pony wasn't too hot & there was some air flowing through the trailer by leaving a back top door open & also the air vent.
If its just for a day trip for a competition I tend to do 2 hours max (although that's mainly for my sanity! )
It took my 5 hours to drive to get my boy to Rockley and we stopped for 20mins about 3 hrs in. He never travels but was fine in a box I hired. I think its important to weigh up making their time in the box as short as possible, against the benefit of stopping for breaks. Having said that, I wouldn't do more than a couple of hours if it wasn't important.
Short Breaks every 2 hours to offer water, meal time breaks for humans, plus over night for 8 hours in stabling. Personally wouldn't go for much longer than 4 days.
There are legal restrictions for commercial transporters which would be a food basis (sorry not sure what they are!) But equally it will depend on the individual animal. Some travel well but get stressed when stopped. With my horse I would do 2ish hrs then break but extend that a bit if trouble free motorway driving. She travels very very well and also seems to appreciate the stop and doesn't get impatient.