As I demonstrated a few posts down I enjoy thinking of really big numbers. This includes thinking about eternity, which does come up when thinking about the afterlife, or life extension stuff. This is hardly a scientific survey, but I find that if I ever end up talking bout transhumanist stuff people often treat immortality as quite a short amount of time, like a thousand or a million years, which is really, really nothing like eternity.
I've never spent any time thinking of a way of trying to describe a long period of time in an intuitive way, though for ages I've like this example.
Say there's a giant iron ball the size of the earth and a fly lands on it an takes off every million years. When that ball has worn away to nothing eternity hasn't even begun yet.
While this doesn't really lend itself to easy retelling (and though it's really about boredom), this is much better.