According to the link within Cancel asking to close? we can not rescind our close votes.
There was a popular question recently (I won't link to it because I don't want to be seen as rep hunting) that received 4 close votes. It teetered on the precipice for a few days, and managed to stay open. When viewing the question now (about a month later) the close votes are no longer present.
So, my question is: do close votes expire?