Once or twice a day, with a rest day once or twice a week is perfect for us. It's what we want and what we do.
The more you have sex, the more you want it and the better you get at endurance. My Man is in his early 50s and can still have sex 3 or 4 times in a 48 hour period. We do have rest days, not scheduled, but when we feel tired or sore. It works out really well for us, as we both have really high sex drives.