This is impossible to say, as it depends on your music and your DAC. Both affect the loudness coming from the headphones. Buy an SPL meter that measures in dBC and measure the loudness yourself. Here is an example. Stick the mic inside the cup, roughly where the opening to your ear is when you wear the HD600. Cover it: Use your hand, a pillow, or something to make a complete seal around the earpad while the mic is inside the cup. That will give you an idea.
Below 85dBC is safe indefinitely, 85dBC for 8 hours is safe, 88dBC for 4 hours is safe, 90dBC for 1 hour is safe.