At least twice a week. I love ACV rinses so when I was wearing twists, I did one daily. I needed to get my hair wet for the twists to hang straight down - I'd let them drip dry that way. So I'd do a CW and rinse with water then ACV rinse. Or I would just make the solution in a basin and dunk my head till twists were saturated then drape a towel around shoulders and let my twists drip dry. I even used to spritz my hair with a solution of ACV, EOs of lavendar, rosemary and geranium, glycerin and distilled water. This was daily so in a way, I ACV'd daily.
BTW, the solutions I've been reading about are way stronger than mine, which could explain why you guys are getting drying effects. I put about a 1/4 cup in a full basin of water and I've never had problems. When I used to use a gallon container to make my solution and pour it over my head, I would use 2 tablespoons (about 1/8 of a cup) of ACV then fill the gallon container with water.
Too much acidity can be bad for your hair, so the way I look at it, even the smallest drop of ACV makes the solution acidic if the water is filtered or soft (which is what you want), and if the water is hard, that little drop neutralizes it a touch making it more acidic than it was (again that's still better than using the hard water). So better to err on the side of having the solution too dilute that to damage the proteins in the hair by having too conc a solution.