  It is well-known that the chances of success of SETI depend on the longevity
of technological civilizations or, more broadly, on the duration of the signs
of their existence, or technosignatures. Here, we re-examine this general tenet
in more detail, and we show that its broader implications were not given the
proper significance. In particular, an often overlooked aspect is that the
duration of a technosignature is in principle almost entirely separable from
the age of the civilization that produces it. We propose a classification
scheme of technosignatures based on their duration and, using Monte Carlo
simulations, we show that, given an initial generic distribution of Galactic
technosignatures, only the ones with the longest duration are likely to be
detected. This tells us, among other things, that looking for a large number of
short-lived technosignatures is a weaker observational strategy than focusing
the search on a few long-lived ones. It also suggests to abandon any
anthropocentric bias in approaching the question of extraterrestrial
intelligence. We finally give some ideas of possible pathways that can lead to
the establishment of long-lived technosignatures.
