How often you update your family tree depends on your level of interest. The more committed you are to finding lots of relatives, the more time you should plan to spend doing research through your family tree builder. It can be very time-consuming!
However, if your family tree has reached a stage where you’re pretty happy with it, and don’t feel a burning need to grow it any further, then you don’t need to update it at all (or only when you’re in the mood). You can just open it up every now and then at family gatherings, and show your cousins, aunts, and uncles all the cool discoveries you’ve made!