Typical life-expectancy in the Culture in one's original body is about 300-400 years. Beyond that point, it's considered normal to choose either auto-euthenisation (approximately 50% of citizens choose this option) or a shift into some form of functional immortality either through the alteration of one's physical body, storage, a shift into a robot body, a shift into an entirely new physical form or (at the extreme end) sublimation.
Every Culture habitat - whether it was an Orbital or other large
structure, a ship, a Rock, or a planet - possessed Storage facilities.
Storage was where some people went when they had reached a certain
age, or if they had just grown tired of living. It was one of the
choices that Culture humans faced towards the end of their
artificially extended three-and-a-half to four centuries of life. They
could opt for rejuvenation and/or complete immortality, they could
become part of a group mind, they could simply die when the time came,
they could transfer out of the Culture altogether, bravely accepting
one of the open but essentially inscrutable invitations left by
certain Elder civilisations, or they could go into Storage, with
whatever revival criterion they desired.
There's also the option of simply having a ship or hub mind alter your body and mind to cope with actual immortality. One of the characters is a man who's been effectively immortal in his natural form for nearly 10,000 years.
He was around ten thousand years ago, at the time of the negotiations
which gave rise to the Culture in the first place. This individual was
named as perhaps being able to help provide proof that what was
claimed in the message to the Gzilt was actually true.
So, not long dissolved in some group-mind, then. Stored, I take it?
Not Stored. In fact, never Stored. Still with us, still alive, still
extant and functioning, twenty-five to thirty full lifetimes after
you’d have expected any ordinary humanoid mortal to have decently
abandoned the corporeal. Indeed, longer-living than any known still
independent Mind or even high-level AI from the time. Like the
fucker’s decided to outlive everybody or set a record or something.
But alive, somewhere, probably still within the Culture.
The Hydrogen Sonata
It was one of the effects of living in a society where people commonly
lived for four centuries and on average bore just over one child each
that there were very few of their young around,
Look to Windward
It's worth noting that for many citizens of the Culture, direct immortality is seen as slightly tacky. A choice that's made by people who're fundamentally immature in some way.
“Sma!” he exclaimed, turning to her. “That’s for you; it isn’t for me.
You think I’m wrong to have my age stabilized; even the chance of
immortality is . . . wrong, to you. Okay, I can see that. In your
society, the way you live your lives, of course it is. You have your
three-fifty, four hundred years, and know you’ll get right to the end
of them; die with your boots off. For me . . . that won’t work.
Use of Weapons
Iain M. Banks discussed this aspect of Culture life in his essay (required reading!) "A Few Notes on the Culture"
Which brings us to the length of those generations, and the fact that
they can be said to exist at all. Humans in the Culture normally live
about three-and-a-half to four centuries. The majority of their lives
consists of a three-century plateau which they reach in what we would
compare to our mid-twenties, after a relatively normal pace of
maturation during childhood, adolescence and early adulthood. They age
very slowly during those three hundred years, then begin to age more
quickly, then they die.
None of this, of course, is compulsory (nothing in the Culture is compulsory). Some people choose biological immortality; others have their personality transcribed into AIs and die happy feeling they continue to exist elsewhere; others again go into Storage, to be woken in more (or less) interesting times, or only every decade, or century, or aeon, or over exponentially increasing intervals, or only when it looks like something really different is happening...