In Marvel comics both Eternals and Asgardians have very large average lifespans. I would like to know which species has a greater lifespan and the duration or time of that lifespan.
ETERNALS LIVE FOREVER
In Jack Kirby's original Eternals run, it is repeatedly stated that Eternals CANNOT die in any way - hence eternal. Since Asgardians are shown to age, albeit very slowly, if the universe lasts long enough, an Eternal will outlive any Asgardian.
However, in later comics, some (even all) Eternals were killed, despite Kirby's assurance that such as thing is impossible. Still, it is probably safe to assume that they cannot die of old age - or at least, so they believe. With no official statement from Marvel about Eternal longevity, this is probably as close as we can get to an answer.