I’ve thought the casinos write at least 1 or 2 year cookie as they claim lifetime revenue share. But even if they write 1 month cookies if players visit the casino every month their cookies should be re-written (and prolonged) for the next 1 month.
I have been promoting an aff. program in another industry that pays % of revenue for the 6-7 y.o. clients. They claim to keep 10 year cookie! I think online casinos should do something the same. But some of them even change aff. link code every year.