Someone here mentioned that using 1tsp ACV per cup of water makes the pH of the solution match the natural pH of hair. So, at that concentration, it could be safely used every wash, since it would normalize the hair's pH. I prefer the results I get from using 1 Tbsp per cup of water in my once a week ACV rinse though. I cowash, rinse, apply ACV rinse (and leave it in), apply my RO, then rinse and proceed normally with the rest of my routine.
CG since 11/2011
2c /3a F /Mii, low porosity

Autumn Products /Routine:
Clarifying: ACV rinses, Suave Professional Men Deep Clean Peppermint
Low Poo: Renpure Luxurious Argan Oil
Co-washes: VO5 Kiwi Lime Squeeze or Strawberries & Cream
RO: VO5 KLS or S&C, Cure Care
LI (if needed): VO5 S&C, VO5 KLS, Cure Care
Stylers: FSG, L'Oreal Studio Clean gel, Tresemme Flawless Curls Bouncy Curls gel
PT: IAGirl's as needed