Well obviously they die, they couldn't be immortal. =P
The way I see it is that most Pokemon have a lifespan a little less than a human's because they mature quicker (I see Pokemon reaching full maturity, regardless of evolution stages, by age six or seven.) It all depends though. Bug Pokemon could have shorter lifespans, like around fifty years, while it's conceivable that dragons live to a hundred.
Legendaries on the other hand...it all depends on whether you see them as unique or not; and godlike, or not. I see them as unique, but still mortal, so I put their lifespan around 500 years, and whenever one dies, a new one of that species is born. (Not reincarnation, though, a completely new individual.)
~Chibi~;249;;rukario;