I am having the same problem and wasn't sure it was a bug from the update. In the game that I lost, I had a musical sim and she wrote the songs on guitar, and it didn't take very long at all. I would set her to write song, then she'd still be playing guitar, then I'd click on the action and it would now say "play guitar", but the song count had gone up. So it seemed to happen really fast. Maybe it was to do with the traits she had?
In my current game, I have a teen sim doing that aspiration on violin. He keeps getting stuck when I try to cancel it (but have found clicking something else stops it ie take a shower). I read a couple of days ago that an action has a circle around it, which slowly fills up to show you how far through it you are (not sure if all actions, or just ones like write song, write book etc). He was playing for ages and the circle barely filled up, so I kept cancelling it. I will have to check his inventory to see if there is sheet music in there. 10-12 hours to write a song is a bit unreasonable when their needs go down and they have to go to school or work and sleep, so you would think you'd be able to continue writing song, just like how you can continue writing books or programming etc.