I think 400-600 years is about spot-on for the lifespan of an average Marine. The Great Wolf and Dante are the exceptions. The Blood Angels have always had a lifespan that was generally longer than the rest thanks to their regenerative sarcophagi. (Although Ive been a Blood Angels ever since the early days of wh40K Rogue Trader, ive always found the whole sarcopagus/actual blood drinking element to be a bit silly ever since it was introduced during the second or third edition.)
An example for a marine retiring due to old age can be found in Graham McNeill's latest Ultramarine novel; The Chapter's Due, where a veteran is made regent of an agricultural colony planet. They don't specify his age but it's clear he's not 'methusulah-age-old'. I think the half-millennium age is a good one.
Don't forget that in the great Crusade, even the oldest marine is just about a few hundred years old as the Great Crusade has only been going on a bit over two centuries and only some veterans are older, being veterans of the Unification Wars on Earth.
Don't take Chaos Marines as a reference because they're infused with Chaos and time spent in the warp is beyond relative anyway. The only 'marines' older than Chapter Master Dante are in Dreadnought form, living corpses permanently slaved to the machine.
Indeed, forget about the whole 'immortality' thing, that's mostly used in an existential way in each single marine being a great hero akin to the Ancient greek heroes like Hercules, Paris, Achilles and the like, ie: beings that are larger than life and are remembered as such by the average Imperial peasant and citizen.