I initially mix and master at a lower level.  I keep levels around 80-90dbs.  I listen 10-15 minutes at these levels, then take a 5-minute break.  I will do this for between 2-4 hours, then I take a full two-hour break.

When it comes time for the finalization (loud listening @ 105-110dbs) I NEVER go more than 2-3 minutes continuous and I pretty much split time.  If I listen to loud music for 5 minutes, I take a 5 minute break before starting up again.

Above is what I've doing since 1999 and it works for me.  Maybe you can go 100dbs for 8 hours a day and never have a problem, but I'm not risking it.  105-110dbs for 30 minutes continuous ruined my ears pretty good, and I had to make sure they didn't get any worse. 

Tinnitus isn't fun.  I might be going overboard a bit with my silence rests, but it's working for me.  Better safe than sorry with tinnitus because it lasts A LIFETIME!