But I think the duration will depends on how their casino would remain profitable. So if they manage to lure more gamblers in their casino the more profitable they are and with that they can sustain everything needed like to sustain their campaign for long term. There are lots of successful casinos in Bitcointalk that's why they should try unique way to promote their site or maybe they became at least be so generous giving bonuses to their gamblers since this could attract lots of people to comeback and play.
The duration will also depend on how much money they allocate to have the campaigns. If they see the result give a good progress, they will allocate more money to continue the campaign or launch the next promo.
No doubt many successful casinos in Bitcointalk because they can running the campaign very well. I agree that casinos need to find which promotion that will runs good for them. But signature campaign is the most campaign that many casinos use to attract people attention.
From what I've seen in the past which I believe is still happening even in the present, signature campaigns doesn't start paying off immediately a casino launches it, casinos will need to sacrifice atleast, 8 weeks which is two months before they can start seeing the good effect of running a signature ad campaign, and the longer they allow the campaign to run, the better for the casino business because more people gets to learn about the casino and possibly join.
And while running a signature ad campaign, it's always advisable to supplement it with some form of promotions or contest, like betting contest, slot contest, deposit contest and so on, all of this helps to make the signature ad campaign even much more effective over time.