From Sennheiser rep. (& tec. support UK), John Willett, postings on head-fi on Sennheiser HD800. He posted the same information on a few threads.
Here is one from 'Break-in for HD800's?' thread:
"The designer suggested 48-hours with pink noise.
Reading these forums - the consensus appears to be between 100 and 150 hours."
Here is a link to another posting from John Willett on another forum re: burn in of HD800 (see last posting on the page):
"Just remember they need burning in - the designer recommended 48 hours on pink noise and many users seem to find 150 hours on average with music.
Edited by SleepyOne - 5/26/10 at 8:51am
They do also need a good amplifier - I use the Grace m902B with mine, but I have heard the Lehmann Black Cube and DACS Headmaster also recommended."