Most people would say that you should do a full wardrobe clean-out at least once a year. This means going through all of your clothes, deciding what you want to keep and what you want to get rid of. You can either donate or sell the items that you don't want anymore.
However, some people prefer to do smaller wardrobe clean-outs more often. For example, every season you could go through your clothes and get rid of anything that you haven't worn in a while. This is a good way to keep your wardrobe fresh and up-to-date without having to do a huge purge all at once.
Ultimately, it's up to you how often you want to clean out your wardrobe. If it feels like it's getting too cluttered, then it's probably time for a purge. But if you're happy with the way things are, then there's no need to force yourself into getting rid of everything.