How loud is too loud for earbuds and headphones?

A lot of us use earbuds or headphones pretty regularly, whether it's while working out or during a long subway commute or whatever, and most of us barely give our health and our hearing a second thought while choosing a playlist and adjusting the volume. But research shows we should be giving it some thought, as causing permanent hearing damage is easier than you might think. Experts have suggested some listening guidelines based on how loud you like to crank your tunes (these are for earbuds):

• At 80% of the maximum volume you shouldn't listen for longer than 1.2 - 1.5 hours at a time
• At 70% of the max volume you can listen for quite a bit longer: 4.6 - 6.0 hours
• And if you can keep it to 50% of the max or lower you can listen as long as you want

I have no idea what percentage I usually have my iPod set on, but I'm gonna figure it out!