The microorganism-like structures in Martian meteorites, and the prospect of thermal vents warming the oceans of Europa, suggest that our own solar system harbors not one, but three, likely locations for life. The heyday of Martians may be past, and if hypothetical Europan organisms have a day in the Sun, it will probably be in some far-off aeon when the Sun itself is warmer. Still, one win and two possibles in a single solar system makes me think that perhaps a substantial fraction of stars with planets, have had or will have life on one of their worlds. The trend toward complex behavior is broad in the animal kingdom. Horses and harvest mice use tools, as do my friends, the Corvids.
Some birds even make tools.
Then, if you take a snapshot of the universe, you see last year's jackpots, plus the year before's, and the year before that's, and so on back to 1000 years ago. The only data we have, is us. We have been capable in principle of radio communication at interstellar distance for less than a hundred years, and during much of that time it looked like we would blow ourselves up in atomic war by a week from Tuesday. If L is less than 100 years, technological civilizations are scarce, and most will be gone before we know them. Yet L might be larger in two ways.