I've bought most of my headphones "pre-owned" so most were already burned in. Of the new ones I've bought, my Massdrop Fostex T-XO didn't sound very good at first. I didn't keep track of the time, but after maybe 30-40 hours, they sounded much better and are now one of my favorites, especially for bass heavy music. I've always let new headphones "burn in" for a day or two playing Pandora or something before doing any critical listening, but 300 hours seems ridiculous, especially if they don't improve.
That frog is definitely not worth kissing!
That frog is definitely not worth kissing!