All told, Nintendo has released nearly a dozen titles in the Metroid series, spanning 25 years.
Of course, that's in real-world time. In Metroid time, only about two weeks has apparently passed between all the games, because the protagonist, Samus, looks exactly the same age in all of them. This is in contrast to series like Half-Life (Eli and Kleiner noticeably age between HL1 and HL2) and Metal Gear (the craziest plot they could do next would be Solid Snake maintaining an erection). The seemingly short time span covered by the series is contradicted by the fact that "Samus kills the last Metroid" is an event that happens three separate times, in addition to murdering enemies like Ridley and Kraid several times each.
The anxiety of coming back to life so often makes poor Kraid overeat.
We might have a better idea of how much time passes if the series' canon wasn't a Gordian Knot of discontinuity. The original 1986 Metroid (and 2004's Metroid: Zero Mission, which is just a tarted-up version of that game) comes first in the chronology, but after that, it quickly devolves into chaos. The Metroid Prime trilogy (2002-2007) allegedly takes place between Metroid and Metroid 2 (1991) -- except that, no, that can't be right, because in Metroid Prime there are log entries that refer to the destruction of the planet Zebes, and that doesn't happen until Metroid 2's direct sequel, Super Metroid (1994). This same bit of information about Zebes is reiterated in the official Metroid Prime comic, where Samus is also surprised to find a live Metroid ... even though she doesn't kill all the Metroids for the first time until Super Metroid.
How can we enjoy Metroid Prime Pinball unless we know where it stands in the canon?!