It's primarily a mamtter of preference and lifestyle. Naturally, someone who leads a rather transient life, moving frequently, may want to postpone such a decision. On the other hand, if you believe that you're residing in the community where you intend to live and die, the sooner you make burial arrangements the better, as with any matter where your family is concerned. Delay can mean that your survivors will be pressed to make a snap decision and pay a higher price for a burial plot when it is needed quickly. There are a number of important considerations when choosing a cemetery.
Some religions limit people of their faith to cemeteries devoted specifically to that faith.
You may want to consider locations where other members or your family are buried. If you have a preference, it's in your and your family's best interest to make your decision in advance.