I wear waterproof over-trousers and an over-jacket that I get about 4 months use from in winter before they leak bad enough to replace. I find they start leaking first at the crotch, then the front of the shoulders and elbows. Visually they look fine, no wear or tears etc. When new water beads off them but after a few months they do start to wet out, so although they are some sort of plastic they must also have a coating.
Rather than replace yet another set I recently tried the 2 stage Nikwax, first is a "tech wash" then the wash in treatment is done after that. I did my latest leaky set plus my previous ones in one go and currently using the older previous set so far so good... Rode in the heavy rain the other day and was bone dry again. They don't wet out anymore but the water doesn't bead off quite as readily which suggests the coating isn't as good as the factory fresh one.
The pack I got was 300ml of each (£9.95 for the two bottle pack) and you only use a couple of cap fulls of each so probably good for 3 treatments.