Once you select the best crystal (or crystals) for your needs, it won't serve you to let it sit on your desk and collect dust. It's important that you maintain your crystal like an engine — otherwise, how else will it continue to run properly?
Some varieties of crystals are believed to soak up the energy around them, while others act as a magnet for energy. For example, protective crystals absorb negativity while prosperity crystals attract positivity and success. Whether they function like a sponge or a filter, they're bound to get a little clogged, so to speak, after a while. That's when it's time to cleanse.
There's no exact rule for when and how often you should cleanse your crystals. As you form a relationship with the stones in your collection, you'll be able to recognize when each one is starting to lag in its powers. If you have a black tourmaline, for instance, and notice that you're having a streak of bad moods, it's probably time to purify that stone, to get it back in tip-top, bad-vibe-blocking shape.