It depends on factors such as the type of mulch used, weather conditions, and the decomposition rate of the material. In general, it's a good idea to check the mulch layer periodically and add more as needed. For wood chips, an annual top-up, usually in the fall, can help maintain an adequate mulch layer. However, if you notice the mulch decomposing more quickly or if there are visible signs of it breaking down, adding a layer more frequently, perhaps every six months, could be beneficial. Remember, the primary goal is to keep a consistent layer of mulch to suppress weeds, retain moisture, and gradually improve the soil. Adjust the replenishment schedule based on the specific conditions of your garden.