The more often you can work your horses, the better. Of course, it is important to remember to gradually work them up to it, and if they have a particularly strenuous workout, give them a lighter day or a day off the next day.
My horses are worked almost seven days a week I can, usually it is not driving, but if I could I would!
Horses are made to be pure muscle and constantly be moving. Standing stalled day after day is counterproductive to what they were designed for.
Andrea