I reproof the Velez trousers with that wash-in nicwax stuff a couple of times a year, after washing them with techwash. But I don't really wash them in between times, just give them a rinse through, same as my other Paramo stuff.
I admit that I am not much of a bad weather walker if I can help it. However, I live in NW Scotland and like doing long distance trails, so my waterproof gear does get tested. And I do like to keep dry and warm.