As an audio engineer it is important to take care of your most precious piece of equipment - your ears.  Exposure to loud music is not much of a problem at short durations.  Sustained listening at loud levels will not only damage your ears but also will blow your perspective (see artical on perspective).  You won't feel immediate impacts but you are effectively shortening your career.

Due to the Fletcher-Munsen "Equal Loudness" curve, music sounds different at different volumes.  When you listen at low levels, you hear less low and high frequencies.  Some stereos have a "loudness" button whose purpose is to make music sound louder when listening quietly.  (Many people leave this engaged even when listening at loud levels for the "smiley" eq sound.)  Since people will be listening to your mixes on different systems and at varying levels, it is important that you evaluate your mix in the same ways.  You can do the varying levels easily in the control room even if you have only one set of monitors.  Get in the habbit of changing your volume continuously as you mix.

So what is a good "normal" or "average" setting?  Around 85db is a good level that allows you to hear well what is going on in the low range without risking damaging your hearing over long sessions.  You will still want to take a break to rest your ears every 3-4 hours without fail.  Not only do you lower risk of hearing loss, you will refresh your perspective and decrease the chance of getting into a rut that is sending you down the wrong path with your mix.

I recommend you calibrate your system for 85db so it is your default listening level.  Then come down from there to 70db and see what happens to your mix.  Does the bass or hi-hat dissappear?  You may need to add mor upper mids (or use a plug-in like Waves MaxxBass) to help your bass stay in place.  (A similar effect happens in the car when the road noise covers up the low frequencies.)  Or you may need to eq your hat a little differently or bring up the level.  I often find that overheads and cymbal tracks can be louder and when you turn it back up it sounds better.  Mixing at loud levels we tend to mix overheads too low.  Now bring it up to 90db and listen to your lows, low mids and midrange.  You don't need a lot of time to hear problems when you jump from 70 to 90db!  Is it muddy or boomy?  Clean it up, then bring the volume back down to about 80db.  Do the kick and bass have proper separation?

In summary, use the tools you have in your room to get as many different perspectives as possible.  Listen in mono.  Listen while standing in the corner or outside the room with the door open.  Listen on various monitors.  And listen at varying levels.  Its all about finding new ways to hear what you aren't already hearing.  If you never hear it you can never fix it.