That long "shelf life" means conservation attempts for unknown, threatened species could come much too late, they said, due to a lack of experts, funding and resources needed to do the job of species identification.
"Species new to science are almost never recognized as such in the field," said Benoit Fontaine of Museum National d'Histoire Naturelle in Paris.
"Our study explains why it often happens that we describe species which were collected alive decades ago and which can be extinct now -- just as astronomers study the light of stars which do not exist anymore."
Fontaine and fellow researchers calculated shelf life based on a random sample of 600 species described in the year 2007 and found those species had a shelf life of 20.7 years on average.
"Our knowledge of biodiversity is still very scarce," Fontaine said. "Describing new species is -- or should be -- part of the everyday work of taxonomists, and we need to hurry; new species are disappearing faster than we can describe them."
The findings were published in the journal Current Biology.