How far back can you change a vote?
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Not far enough. :(
It's a very short time, not specified exactly so as to prevent gaming, but it's somewhere in the realm of 1 or 2 minutes after the post was last edited.
If you have a high enough reputation, you can remove or change your vote on an old post by editing it first. Jeff has stated that this is considered acceptable, but personally I would prefer we just grant this ability directly.