Right; the parameter that would be of interest is rather L = the length of time for which such civilizations release detectable signals into space. Quoting Wik

Right; the parameter that would be of interest is rather L = the length of time for which such civilizations release detectable signals into space. Quoting Wikipedia: "Inserting the above minimum numbers into the equation gives a minimum N of 20. Inserting the maximum numbers gives a maximum of 50,000,000. Drake states that given the uncertainties, the original meeting concluded that N ≈ L, and there were probably between 1000 and 100,000,000 planets with civilizations in the Milky Way Galaxy." As I understand it, at the time it was estimated that a civilization would broadcast during its existence, from the time radio communications started until the fall of civilization (thus the Great Filter). Now, from our sample size of one, it looks like L would rather be on the order of 100 years (in our case, not because we are trying to hide in the Dark Forest, but because we don't want to waste energy beaming Dallas reruns into space for no good reason).