Not to fuddle about much but...  The problem is we have met the enemy and he
is us.  For any resource of quantity R, a per-capita demand (that's you) of
d per year will tell you easily how long the resource will last depending on
how many d's there are.  Demand units are (d) per folk.  Multiply the number
of folk N (that's us) by the demand (d) and you have Nxd.  This is the
drawdown on the resource R.  EG:  If d is 20 sheets a day of toilet tissue
and there are 100 of us that's 100x20 or 2000 TP per day.  If the defined
ultimate TP resource is 2000x1000 or 2M then in one thousand days you will
be wiping with ... insert favorite wiping device here....instead of what the
bears think is the very best quality...

EG: If there are 100 instances of a particular wear item on your Vanagon in
stock throughout the world and no more are being manufactured and the demand
is 50 per year and the life of the part is 5 years.  How many should you
squirrel away (sorry mike) to keep your vanagon on the road for ten years
and when should you do it?  2? RIGHT NOW! now how many future thinkers are
there like you 25? 50? 100?...you get the drill.  Get ready Ron...

Do the math.  The model is simple.  Now fearlessly apply it to your present
parameter space.  Shocking, eh?  But you knew that, right?  Bumping up
against reality can be a sobering experience.  Or...you could sell the van
to BenT.