@Tammy91, @Matthew285 - Since there is no information exactly about this topic, my thoughts are speculative and based on other information known about the Superhost program.
They do not update it frequently since the statistic is not exactly a part of the Superhost criteria. It is a limiting factor, meaning, if you don't get reviewed by at least 50% of your guests, then you won't be considered regardless of the other factors. It's to encourage you as a host to write reviews for your guests, because typically, when one writes one, the other writes one. It will also encourage you, as the host, to reach out to guests after the reservation is over to complete one if you find your clients are not being active about completing this. It's also to help eliminate hosts who meet the minimum 10 reservations a year but get only 1 review. It doesn't give a very accurate picture of the host based on one review every 12 months.
And since it is based on a year's worth of reviews, it's likely they only update that number once a quarter, but there is not any notation as to the frequency of the update. Ultimately, you should have a pretty good feeling of the number of reviews written by your guests to ensure that you meet the 50%
https://www.airbnb.com/help/article/833/what-s-the-superhost-section-of-my-dashboard
https://www.airbnb.com/help/article/829/how-do-i-become-a-superhost