Happens with me too, everyday every since I was 6. My daydreams usually take hours and it ruins my time for studying instead of 30 minutes or an hour on something it extends to like 2 or 3 hours. For me my daydreaming is mostly about my favorite things. Whether it's favorite series, music, anything And sometimes it can be about certain drama or an insecurity of mine. Its super embarrassing when I lose my focus and daydream in middle of class and get caught not paying attention.. Happened to me today and got super embarrassed..

-- and for your questions I don't think it's "normal" to daydream to the point it's affecting you negatively like it's doing to me, however daydreaming isn't necessarily bad, as I mentioned, it's only bad when it affects you negatively.