Poster Design

Hi, I designed 2 posters for a school. At first, it took me 3 hours to do and I thought there would only be a couple of small revisions.
But then they decided to change the whole thing and there was lots of back and forth. I ended up spending at least 12 hours on it and everything took much longer because it was huge files!
Originally we made up $200 for both but I told them I’m going up. How much should I be charging them for this?

I’d appreciate some insight on this.
Thank you