It depends on the treatment the horse receives and the problem that needed attention. We give our horses a full day of rest after an adjustment. Ours are fit racehorses though, so the situation is different if like you said your horse has not been in much work. Your chiro will tell you the best thing for your situation, but in my experience with our bone cracker, he suggests 24 hrs of rest after treatment.
Good luck - I love chiropractic for my horses and for me too!