I have been waiting 15+ hours now.

I think it depends on the UAs. If they have time to approve, they do it, if not, you have to be patient.

It's slower on weekends, but the longest I've had to wait is 48hrs or so :)

On the billboards page, it says: "All billboards must be approved by a staff member. This can take up to 24-48 hours."

So it might be approved this weekend, but it may take until Monday.