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

Winter Products /Routine:
Clarifying: Herbal Essences Tealightfully Clean
Low Poo: Renpure Luxurious Argan Oil
Co-washes: Suave Naturals Tropical Coconut or Daily Clarifying
RO:
Tresemme Expert Selection Perfectly (un)Done, Cure Care, Suave Tropical Coconut or Daily Clarifying
LI (if needed):
Tresemme Perfectly (un)Done, Renpure Brazilian Keratin Straightening (old formula), Cure Care
Stylers: HETT, AIF
PT: IAGirl's as needed