I saw stuff in there I downloaded days or maybe even weeks ago. (Couldn't tell you exactly when since I've deleted it all now.) I've heard reports of people seeing downloads from years ago.

Here is a SQL to view timestamps:

SELECT datetime(LSQuarantineTimeStamp + 978307200, "unixepoch", "localtime") as LSQuarantineTimeStamp from LSQuarantineEvent

(Based on http://www.forensicswiki.org/wiki/Mac_OS_X with modification.)

In my testing, only downloads from Safari are logged here, not those from Firefox (haven't tested Chrome). If you don't use Safari as your primary browser, that might explain not seeing many downloads.

No, it's permanent. Try the app and I think you'll see a longer history.