I’ve heard it differently, for casino websites is
sum of last 8 months revenue of the website.
last 8 months, as their is an attrition rate of players – eg new players are needed to replenish the ones that have left the casinos you sent them too.
the last 24 months does not give an accurate value of what future earnings could be – as the time frame is too long. Whereas the last 8 months performance of a casino website gives a closer representation of how it has been performing.
Saying that, I like to use other metrics as well, such as keywords, age, domain name, content, serp footprint etc