SPMU can last several years however a top up is recommended around every 12 months. Anything over 18 months will revert back to the original price. Top ups can be carried out sooner that the 12 month period if needed.  This  depends on the client and how they well they retain the pigment.  Also how perfectly groomed they like the area to apppear. Factors that can contribute to fading are frequent sun usage, frequent swimming, chemical peels. Basically anything that is harsh or abrasive to the treated area.