An image processing apparatus that is capable of calculating an appropriate replacement guide value of a part even in a case where the image processing apparatus operates under different operating modes. The image processing apparatus is capable of operating in a plurality of operating modes of different image forming speeds and has a replacement guide default value showing a guide for replacement of a part for each of the operating modes. A degree of wear of the part for each of the operating modes is calculated based on the replacement guide default value. The calculated degree of wear is, in a case where the operating mode is changed, converted to a degree of wear for the operating mode after the change, and a replacement guide value of the part in the operating mode is calculated after the change based on the converted degree of wear.