Here's what I have in our Kava Glossary:
RT (Reverse Tolerance)
Reverse tolerance is defined by the kava community as the "break in" period where the new kava consumer may not feel any effects at all from kava. It is theorized that for some people a certain level of kavalactone buildup in the body must be present before one begins to feel the full effects from kava. The range of time it may take to overcome the reverse tolerance ranges per individual, from weeks to up to a month and a half. Some individuals feel the effects immediately.
It took me a good 3 to 4 weeks of use before I felt anything from kava. Then it hit me pretty hard when it did. I believe it's a physical thing that vary's with each person, but I also think that there is a mental aspect to it as well when a new user has no idea what to expect until they have a little experience. I also believe that the products you first use and how you prep and use them makes a difference too. If you buy crappy kava or don't know how to properly consume it, will make a difference too.