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I believe it just means how long the entry stayed on the front page. There shouldn't be anything special about computing this number.

Ah, that makes sense. I'd initially interpreted it as some kind of engagement metric.

yep - I missed that concept too.

"...ordered by {time spent there}->(how long it was on the front page)"

Yes, I see how that was ambiguous. Have changed the language—is it clearer now?

If anyone can figure out how to make it shorter, let me know. It feels a bit wordy now.