I have 7 cats so it's hard to explain when I do it. Whenever I bring a new kitty in, I test right away, and I also test a few more times after that, just to be sure. Usually throughout the course of the next 2 years. Although, my Cajun is 10 years old, and I even tested him last year. It was probably his 8th test or so..
But then, I get my vet care for free and I don't think about it, I just do it. I have their bloodwork done every year, for eveyone, even the young ones, and I check their urine too. Yes, it's perks of working at a vet clinic, but even if I didn't I'd still do those tests.