SMP is a permanent solution for hair loss, however for the average client touch ups will be needed every 2-4 years to ensure you look your best. It really does depend on a lot of factors (lifestyle, immune system, exposure to sun, age, etc.), and of course their preference. If a client is happy with gentle fading then every 5-6 years is fine. If you prefer a crisper look with a tighter edge up then every 2-4 years might work better.