When it is no longer effective or when it stinks too much from your sweat. When these happen depends on your own circumstances.
There is no convenient answer like “replace every ten years,” you see. A memory foam topper is supposedly able to last ten to twenty years of normal use. However, how old is the memory foam mattress pad that is delivered to you from the warehouse? Is it brand new? Or is it already five years old? Or maybe even ten years old? Did the vacuum packing hold out properly? Was it stored in a cool, dark part of the warehouse or was it exposed to the sun or cold winter draughts? Are you underweight or overweight? These are all factors which can affect the rate at which your memory foam pad deteriorates.
As for your sweat, keep in mind that a memory foam bed topper is rather warm to sleep on. It does not let air and heat through, so you will sweat more. If you do not protect your topper with a cover or a second bed sheet, your sweat will soak into it. And because there is no practical way for you to wash your topper, it will stink sooner or later.
As they say, your mileage may vary. There is no way to tell you when to replace your memory foam bed topper.