The stackexchange software currently imposes a cutoff on changing votes, i.e. a vote cannot be changed after a certain time has elapsed. I can understand the rationale behind having such a policy as long as the "grace period" for vote changes is reasonably long, e.g. a few days. However, I recently found that I could not change a vote on a question after only 9 minutes(!). This time limit seems rather unreasonable, since (as in this particular case) it is easy/common to misread/misinterpret a question. Is there a good reason to have only such a short window to reconsider a vote? or is it practical to increase the allowed time? I feel like it does a disservice to the question asker to be unable to retract a downvote.
The time limit to retract a vote is 5 minutes. However, as mention in a comment, if the question is edited all votes are "unlocked" and can be taken back or changed indefinitely.
If the question was sufficiently unclear for somebody to get confused, likely there is room for clarification via an edit. After this was done, by OP, voter, or whoever, all votes can be changed.
In corner-case, like "mis-click"-votes etc., there is the possibility to make some irrelevant edit for the purpose of unlocking votes. However, I would advise to try to minimize this, and to hold back (down)votes when in doubt, and to ask for clarification first.
A reason for having a short-grace period is precisely to preempt against "tentative votes;" if you are not (yet) sure what to vote, just don't vote (yet).