How often should I clear the cache?

I don't see how that can possibly be related to the cache. The cache contains files from websites, like images, stylesheets and scripts. So when you visit a page you've visited before, you don't need to download the whole thing again. The cache therefore allows pages to load faster, not slower. The cache has a fixed size, so files are deleted automatically as new ones are cached. There's no need to [[How to clear the Firefox cache|clear it manually]], unless you've having some sort of issue like [[Websites look wrong or appear differently than they should|websites looking wrong]].