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It doesn't work like that. Think of your vote as a standing endorsement that can be added or removed at ANY time. There are no fixed terms although technically the Top 20 are scheduled every 1 minute, so maybe you could say it's a 1 minute term :)

Witnesses in the Top 20 (at any time) get regular blocks to mine and have a greater weight in policy matters, others outside the Top 20 get a lot less.