Many people look at getting a memory foam mattress topper instead of spending so much money on a memory foam mattress. For a small fraction of the cost of a new bed, a memory foam mattress topper works for many as well as the full mattress would. But how long will it last?
No mattress toppers last as long as a full mattress, not even the high-end designers. The reasons for the shorter lifespan are the thickness (much smaller, usually, in a topper) and the overlay issues (if you put a memory foam mattress topper on a sagging mattress, the foam will take the shape of the sag, as well).
Though there are some brands that offer a lengthy warranty, most memory foam mattresses wear out between 2 and 3 years after they are purchased.
You should have no problems getting three years of live out of your memory foam mattress topper is you are sure to get high density, thick memory foam and put it on a reasonably solid foundation (if your mattress is 15 years old, not much can really give it new life).