I agree with a time limit. 6 months or 1 year, max. And some sort of way to prevent "automatically re-voting" using a bot.
I also agree with the limited number of votes. I'd go with 15 to cut down the current number by 50%. Also, it's hard enough to read and keep up with 15 individual blogs, let alone 15 witnesses on top of that.. it will provide for more quality witness voting.