We did this in September at Boma/Jiko. At the time it was 4:00, but certainly check the flyer they give you at check-in to be sure. (Or if you're not a guest at AKL -you can still do it. They have little notecards posted right near the concierge desk when you first walk in to tell you what's going on at the resort, and they also have them downstairs in the waiting area between Boma and Jiko)
We liked the tour, but agree with the previous poster that the Jiko part was better than Boma's. Our tour group was HUGE - probably ~25 people - and it didn't look like they were really prepared for that. We couldn't really hear our tour guide in Boma, but he was mostly just explaining the different cuisines served, and since we eat at Boma all the time, we already knew the information he shared. We certainly didn't mind the soup sample, though! And there were a few interesting tidbits we could hear that made this part worthwhile.
Jiko was fascinating - there were all kinds of design elements in the restaurant that we never would have known if we didn't take the tour, and the bread/dip samples we had were great.
It's a free tour so you can't really complain and it was definitely worthwhile - I just wish our guide had a microphone!