Aside from being reusable, menstrual cups work by collecting your flow, rather than absorbing it. As a result, they can be left in for longer than tampons and don’t leave you feeling as dried out.
Inspect your cup regularly for signs of deterioration such as a sticky or powdery film, severe discoloration or odor, etc. If you detect any of these signs, or if you experience irritation, it's time to replace it! Once a year is a good rule of thumb.