When you put on your hair prosthesis, you might notice some thinning after a while. How long this takes depends on each person.
There are lots of reasons why this happens. If you pick an Ultra HD polyurethane base, the hair is put in using something called the "V-LOOP" system. The hair isn't tied straight into the base, but injected instead, to make it totally invisible and natural-looking.
Rubbing your hair when you style it can also cause it to thin faster. Shampoo, treatments, and dye can be things that make you lose density too.
How long a hair prosthesis lasts only matters once you've put it on. If you keep your hairpiece without using it, you won't see any loss of density. So you can keep your hair prosthesis for years if you don't use it.