I used it in my hair twice in the span of one week, which is probably a little excessive but eh. I put in about 3 oz. per treatment, applying it at the ends first and then working up to my roots. I honestly haven't noticed a huge difference, but I think my hair feels a little "harder" after using it, but in a good way. I have fine hair, and I like for it to not just hang.
I used the Voskos brand, but any brand should do. Also, try to get the 2% or 4%, since the fat will be hydrating for the hair. If you intend to buy Greek yogurt especially for use in the hair, Cabot sells a 10% fat version. I would never eat anything with so much fat, but I'm sure my hair would love it!