I must agree with Krystall.
Not sure how long the average player will stay with a casino, but I have seen that there are some casinos where the players seem to return in greater numbers.
When you find a casino you would like to promote, definitely go with the casinos where there is a mention of “getting a bonus every month”.
The reason I say this is that this is one of the first indications that people will actually put some effort into retaining their players after the first time they play.
The list of affiliate programs listed here as certified, would be a good place to start your search. Have a look at the casinos they represent and then look at what is mentioned on their respective websites about monthly bonuses or something of the like.
Best of luck and happy player hunting.