I've noticed it takes about the same amount of time to reduce your inbox by half, regardless of how many emails you start with.
For example, going from Inbox 1000 to Inbox 500 takes about as much time as going from 500 to 250, or from 100 to 50.
This suggests the time it takes to deal with each email follows a Zipf distribution, which seems plausible.
Although intended as a somewhat pithy, anecdotal observation, it's of course empirically testable.