From what I understand, there’s a queue system in place. However, ads won’t be served or paid out unless your node/browser is online. The ad service is also designed as a motivator to keep the browser running. By keeping your Tangled browser open, you’re not only receiving ads - you’re actively running a Millix node that helps validate and propagate transactions across the Millix network. And your node gets paid for every transaction it helps validate and propagate.