Depends on what you are doing during "on". Although generally I don't give much credence to "rule of thumb". As they tend to get repeated like they are the gospel, with no thought or further research. In the endurance world its one day per 10 miles. IE you do a 50 mile ride the horse gets 5 days off. The logic on this did break down for me though. As the arguments for it tend to involve mutually exclusive opinions.
IE a horse NEEDS, 5 days off after doing a 50,,,
But these same people do 3 day 50's... IE ride 50 a day for 3 days. Which leads me to believe the horse must not have really needed the 5 days off.
But looking at actual data, I went back a few years on a couple rides and see a marked trends that people that do multi day rides tend to have rather drastic fall off in performance on the second day. So obviously there is a needed recoverytime. Maybe one day I will try to come up with a data base and see if it really pans out that way. All in all IMO, if your horse is only getting a hour of two of arena work, you could do that everyday, but its not goona turn into a lazy slob if you give it weekends off.