conunundrum:

The Drake Equation
(sometimes known as: Green Bank equation or the Green Bank Formula)

The Drake Equation is a formula that may calculate the possibility of contactable extra-terrestrial alien species. 

Proposed by Frank Drake in 1961, the Drake Equation is a probability argument that would estimate how many contactable, active and communicable alien species there are in our Milky Way. 

The Drake Equation is, as follows;

N = R_* • f_p • n_e • f_{l •} f_i • f_{c •  L.}

In this equation, N =  the number of civilizations in our galaxy with which communication might be possible. 

• R* = the average rate of star formation in our galaxy.
  
• fp = the fraction of those stars that have planets
  
• ne = the average number of planets that can potentially support life per star that has planets
  
• fl = the fraction of planets that could support life that actually develop life at some point
  
• fi = the fraction of planets with life that actually go on to develop intelligent life (civilizations)
  
• fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
  
• L = the length of time for which such civilizations release detectable signals into space
  

The Drake Equation acts as a summary of which we can expect to communicate (if at all) with those who are extra-terrestrial.  The last four parameters: fl, fi, fc and L, are not known and are very hard to estimate, with values ranging over many orders of magnitude. 

Therefore it is not a direct measurement of when we will communicate but a roadmap towards creating and estimating the means necessary to communicate with our space buddies.