From what I understand, for those that respond, the change is very fast. In my case, I'd say a day or two. I started right off the bat with 6 grams/day over 3 doses all taken w/ grapefruit juice. I now take a couple grams/day maintenance on a sporadic basis.
In my experience, and in my conversations with GaryWary, you should notice pretty quickly. If you haven't noticed over a day or two, try upping the dosage.