It depends. While working on coding or other PC stuff I only can listen for about 3 hours before I get too fatigued. But when out in the woodshop I listen to my earbuds in my hearing protection earmuffs for about 6-8 hrs at a time.
Only and hour or so through headphones. My better setup is in my room, and the computer isn't there, which I need for homework. Through my computer speakers, 3-4 hours a day. They have a very laidback sound, so I tend to be able to tolerate a long listening session.
For me I would only listen to my headphones when I'm really listening to music not when I need some background music(the speakers does this instead). And also when I do not want to wake my brother up from his dreamland