While monofilament nylon strings never need to be changed unless they break, you will find that wrapped strings can get "dead" over time. How much time depends upon the environment where the harp is usually located and how much you play. It would be the most common for wrapped strings to be changed every other year but that is by no means set in stone. 

A Wrapped String Set for your Rees Concert Line Harp contains one of each of the wrapped strings for your specific harp model. If you are unsure of the model include the serial number of your harp, from the lutherie label inside the soundbox, in the notes section of your order along with the total number of strings on your harp.