It's 501 exactly. I just hit this limit last week.
Some place above 472..as I can't down-vote yet (despite being here 3 years).
Yeah, it seems we're expert lurkers. I've been here more than 4.5 years and have less than 350 karma. I feel pretty confident about my downvoting skills, but I've never been allowed to use them.
at least 500 karma, though the threshold may have increased