Given the current rate of usage on average for the entire world, how
long will the KNOWN oil reserves last?

Depends who you ask. You might look at: 

Twilight in the Desert: The Coming Saudi Oil Shock and the World Economy 
What happens if the Saudis cannot satisfy the growing thirst for oil
in the United States and China?  Matthew R. Simmons outlines his fears
and predictions.http://uc.princeton.edu/main/index.php

That depends on how you define reserve.  There are reserves of pooled
oil in the ground.  That is the easy stuff to get.  Then there is also
oil in tar sands and shale oil.  These latter two sources will be
increasingly tapped if the price of oil stays high for a sustained
period of time.

Yes, it depends on who you ask, or better yet, who you believe.
The majority of the known remaining reserves are tightly controlled
and the amount remaining are closely held secrets- often secrets
classified by the relevant government.  Currently estimates vary from
10 to 100 years based on current consumption.  The catch with this is
that consumption will change once it is realized how little is left -
starting with oil hoarding and ending with the collapse of many of the
world's economies.

Thats very comforting. 

Thanks.

Att some point many countries may decide to import oil, rather than
use their own. That way they will still have a supply when others are
depleted. That transition may be the signal that governments have
detected the point when oil will effcetively run out.

The United States Geological Survey estimates that there are enough
petroleum reserves to continue current production rates for 50 to 100
years.