How far back can you change a vote?
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.
3It seems silly to only allow high-rep users the ability to retract old votes. Oct 19, 2009 at 17:47
and a post is cosidered edited after the question votes on a answer? Oct 19, 2009 at 18:02
@homestead No, only after the poster (or someone with sufficient rep) edits the post with the "edit" link (next to "flag", under the post).– EtherOct 19, 2009 at 18:14
@Chris I agree, but Jeff seems to consider it "by design".. :(– EtherOct 19, 2009 at 18:14