I could say a character wakes up sad one day but knows music cheers him up, so he takes out the instruments and plays until he gets tired, and the question would be how much time has passed before the next scene occurs
The answer is another question: "How long is a piece of string?"
To make it clear let me change the subject in your question from playing an instrument to running -
I could say a character wakes up sad one day but knows running cheers him up, so he puts on his running shorts and shoes and runs until he gets tired, and the question would be how much time has passed before the next scene occurs
Posed that way it is very clear that it depends on the person. There are actually 6 day races in which competitors run for 6 days with breaks for eating, drinking and the toilet. Whoever runs furthest in the 6 days wins. A select few people are capable of taking part. At the other extreme there are people who struggle to run for a bus. Or, if you prefer to limit it to professional runners, there are sprinters like Usain Bolt who have to stop after less than 400m. The same is true for musicians.
Obviously the ultra-marathoners don't start with 6 day races or even marathons. They start with much shorter runs and build up. Doing otherwise would lead to immediate injury. The same is true with playing an instrument. If you start off by trying to play for 12 hours you will quickly get injured.
However, as famous violin teacher Leopold Auer used to say "If you are serious, practice three hours a day; four if you are a little stupid." (see Mary Ellen Goree's comment in this thread).