According to this meta post from 2015, changes were made so that all close votes age away after 14 days, and your ability to cast a close vote on a question you already voted to close resets 14 days after your previous vote ages away. In practice, this means you should be cast a new close vote on a given question every 28 days. And this is my experience, and I think it works well.
Except when it isn't and it doesn't. Here is a question I voted to close in 2011:
The close vote aged away, but my ability to vote again to close it has never been restored. As a result, there is an 8 year old, completely unanswerable question which I can't do anything about.
Is this really by design? If not, is there an easy fix? The changes to the close voting requirements on Stack Overflow has given new hope to those of us that lurk on minor and not busy tags that we might actually be able to get questions closed, but this is another unexpected roadblock I just tripped over....