Other than caching every hour, or every week, you may cache according to file data.

Example (in PHP):

<script src="js/my_script.js?v=<?=md5_file('js/my_script.js')?>"></script>

or even use file modification time:

<script src="js/my_script.js?v=<?=filemtime('js/my_script.js')?>"></script>