Assuming you are referring to the original Beautyblender sponge, beauty experts generally recommend replacing your Beautyblender every 3 months. Of course, how often you replace yours may vary depending on how often you use it and how well you take care of it.
If used daily, your Beautyblender will likely need to be replaced more frequently than if only used a few times per week. Additionally, if you take good care of your sponge (i.e., cleaning it regularly with soap and water), it will last longer than if neglected.