That's how long commercial emailers say it may take for an unsubscribe order to take effect. Others say 24 hours.
Non-commercial email list-servs, such as often come free with a cheap domain shared-hosting account, can process an unsub order in milliseconds.
My late mother-in-law was on a lot of commercial mailing lists. Hell, I'm on a lot of them, but I also work at controlling the volume. But now that I'm having her mail forwarded to me so that I can make sure important messages -- from people who haven't gotten the news, or don't know how to contact next-of-kin -- don't sit unnoticed in an unchecked inbox, I'm having to send a lot of unsub orders.
Some outfits use a system that asks why you're unsubscribing. At least one such continued sending well past the advertised waiting period, and I couldn't resist invoking the old "Saturday Night Live" Franco gag. I like to think mom-in-law would have approved.
The volume is subsiding, gradually, but just now I had to tell another persistent shop that if I'm forced to start telling Gmail that their messages are spam, that could have implications beyond just one email address.
As easy as it is to get on these lists, there's no excuse for this slow response time. It's not as if these unsubscribe orders actually need to be processed by a human when a gnu can do it so much more efficiently.
Update: Forgot to mention, some of these businesses make it hard to even submit an unsub order. Those I'm just sending straight to spam from now on.