I was building a demo for one of our clients, and decided to experiment with content target through audiences. I defined an audience called “Executive Management” which had the department set to Executive Management.
I waited for a few hours but no compilation of the audience occurred. I did some research and found this page that states that SharePoint Online compiles audiences once a week!
The Audience Compilation Timer Job for the User Profile Service Application runs weekly on Saturdays at 1 am in the time zone of your data center. You can check the Timer Job Schedule report in the Service Administration portal to see the job’s last run time.
While on premise you can access Central Admin and run this job manually, there is no option in SharePoint Online so plan your audiences carefully and be prepared for slow updates for this feature.