If you are the publisher you need to be aware of the tracking duration offered by the advertisers/merchants with whom you choose to work. We commonly refer to this as Cookie Duration or, simply, Cookies. Some, big brands (mostly?) offer very short or even no tracking gap. That means if a customer you refer doe not make a purchase during that first visit you DO NOT get commissioned for that sale.
While investigating merchants to promote (on my sites and on your sites) we do need to be cognizant of the Cookies being served up to us as part of the merchant-affiliate agreement. 30 days is adequate – anything less favors the merchant and hurts “our” bottom line. In my example from “yesterday” – had the tracking gap been only 15 days – I would have “lost” being commissioned for two out of those three sales. BTW, that last sale had a commission of $50.89.