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.

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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.

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This suggests the time it takes to deal with each email follows a Zipf distribution, which seems plausible.

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Although intended as a somewhat pithy, anecdotal observation, it's of course empirically testable.