L --
the length of time such civilizations
emit electromagnetic signals.
One might argue that this number should
be at least 50 years -- which is what
its been on Earth so far ...
... and in fact, in the early years
of the Drake equation, a pessimistic
number like 50 or 100 years was often
the value used for L. Remember,
this was during the height of the cold
war. [Overhead]