I frequently get 30 seconds of unskippable ads multiple times in a video. One set at the beginning and following sets at approximately 3 minute intervals.
At least some mid-rolls are skippable but they are totally random. Like, maybe a set of 2 ads where the first is 15 second and the second is 15 seconds that is skippable after 5 seconds. The combination of length and skippable is confusing.
More annoying is the tendency to sneak a 3 minute ad in there that is skippable. So unless I am in the room and near the remote it will just play the whole thing. I've had ads that are up to 15 minutes long.
If they would offer me a sign-up deal I'd probably just get YouTube premium at this point.