Assuming you are asking how often someone goes shopping for new dresses as opposed to how often one should go shopping for new dresses, the answer may vary. It depends on many factors such as budget, lifestyle, and frequency of occasions that require a dress.
For example, let's say you are a college student who only attends formal events occasionally and when you do, you prefer to wear a dress you already own rather than buying a new one. In this case, you might only go shopping for new dresses every few years or even less frequently. On the other hand, if you are a working woman who attends work functions and social gatherings regularly, you might find yourself going shopping for new dresses much more often - perhaps even every few months or so.
Ultimately it comes down to personal preference and what works best for your individual lifestyle and budget.