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This won't down vote everything. Votes get throttled, so I guess a setTimeout has to be used.

They also explicitly state that you are to make no more than 30 requests/minute[1], or restated, no more than once every 2 seconds.

That's for the API. I don't think the official web site counts.

They also mention Greasemonkey scripts, which is functionally the same as running script from your console, so I believe it'd still apply.

Either way, it's probably a good guide to follow if nothing else -- I mean, reddit still has near-daily capacity issues.