When training, I give my mare a day off for every 10 miles if we are going moderate or harder. If we just walked, stopped, grazed, walked a little more and putted about, I might ride her the next day. It depends not only on the miles, but the quality of the work you do over those miles. Lots of hills/sand = more rest.
When we race, I give her a solid week off, like nothing at all but hanging in the pasture. If we did a 25, I would go back to my regular training schedule. After a 50, she gets 7-10 days, and then we come back slowly, like 5 miles at a walk, something so easy, and we build back up from there - how much we build and how fast we get back to regular training depends on her - she lets me know how she is feeling and I watch/feel her out carefully.
I taper the workouts the closer we get to the ride, and then rest her coming into the ride a minimum of 5 days, but usually more - it depends on the terrain of the ride.
She is all I have, so I am quick to give her rest if I feel she needs it, and I try to make the most of the work we do when we do it. I get about 30-40 miles a week in when we are training. My goal is 1 race a month until June (and then it is too hot! I ride her, but only early in the AM before the heat gets up, and it is short, arena only just to keep our skills), and we take a mid-summer break until end of August and then ride fall/winter - I hope to do I race a month, then, too. I know, most people take winters off from riding, but down south, winter is great riding weather and summer sucks, so I give my mare her annual break during summer.