I hit a 99 score on the evening of the 7th day, and 99s were supposed to have started getting it that weekend. However, that was the Sunday of the infamous 10.3 rollback, so I didn't expect it right away.
Turned out that I had to wait until I got home from work 5 days later, on Friday afternoon Eastern, before I got the email and update to start using FSD Beta (10.3.1 in the middle of its two week period).
Even though 10.4 came out the following Saturday, my car didn't start downloading it until the wee hours of the morning the following Monday.
So, it seems to me that it's a little bit the luck of the draw, regarding when any one car actually gets FSD Beta, both initially and new versions.