Depends on so many factors. We like to keep our horses fit enough to cope with at least the sort of work load they could cope with in the wild. Depending on your source material, this is typically reported as anything between 10 and 30 miles a day.
20 years ago the horses I looked after would think nothing of doing that sort of mileage - but they did wear shoes and I gave hardly any thought to their feet at all.
The two mares in the photos did five hours today. Probably 3/5 of the surface was the small flints over hard chalk that you can see in the first picture. This was a challenging surface today, they are both experiencing a bit of flare - a few weeks ago the Grey would have had no problem and Madam would have really struggled. So the Grey has gone back a little but Madam is getting better all the time.
The three pictures of them on grass are towards the end of the ride. They are both still feeling very well and ready to go. I am hoping that their new much stricter diet (no grass) will improve their performance still further.