525
YouTube's ad problem just got worse: Users now seeing 90-second unskippable ads!
(www.androidauthority.com)
This is a most excellent place for technology news and articles.
That's one of YouTube's anti-adblocking measures. As far as I know, it only happens on a “cold” load (like when opening a YouTube video by going to its URI directly). You can avoid it by navigating to it from within the YT interface.
Funnily enough, the time you have to wait is still shorter than the ad you would've had to watch.