If we define the perception of time passing through novelty, then there's less novelty the more experiences you have. I think that's an interesting model (if not perfectly accurate).
Likewise, the accumulation of relevant experiences might make time seem to actually pass slower, because you're making more connections at each moment and (arguably, might be false) have a more coherent and active brain -- but if there's less novelty worth remembering, it may appear in retrospect (when you examine your memory) that less (perceptual) time has elapsed for the same wallclock time.
So I think this suggests two kinds of time: instantaneous perceptive time (how much your cognition is active -- for example, it may be near zero during anesthesia) and comparative memory time (how much you judge time intervals from your memory of events).
One interesting example is perhaps when we've had an exciting week and it both seems that a month has elapsed in that week (since so much has happened), and that "time passed so quickly!". The former is explained by comparative memory time; maybe the latter is explained by us simply not thinking about the passage of time at all, and "skipping our clocks" (like forgetting to increment a clock or calendar). This is of course mostly speculation.
Waiting a day doesn't feel nearly as long when you've already waited 18,000.
The experiences of even a novel day have less weight if you have hundreds of similarly novel days to compare against.
Edit: In saying that, if I was looking up at a stack of 80 boxes and someone added another on top it might feel subjectively like the tower was extended by less of an amount than if it was the 3rd box being added.