That policy is ridiculous.  Your vet should not be requesting that you
get an exam and urine analysis every time you need to buy food.  Many
vets will let you buy CD OTC - check around.  If not, get another vet.
Once the cat is diagnosed, you should be able to buy it any time you
need it...and they may request one follow-up to make sure the food is
working.  Other than that, you should be free-to-buy.

-L.

I don't know the potential ramifications
of leaving a cat on C/D-S too long.
There are ramifications to leaving them
on other diets "too long".

You'll have to do your homework, on the
condition, and the potential ramifications
of leaving them on such a diet "too long".

And it may lead to you using a different vet
in the future -- though the goal is not one
that "never" rechecks the cat.

Different vets charge differently even when
repeat testing is necessary. I've experienced
that in the past year. 1,000's of $ in vet
bills didn't stop one vet from charging me
$50.00 a day to run my cat in, not to be seen
by the vet, but to get a shot of antibiotics,
and subcutaneous liquids by the vet tech.
Then recently, a vet I'd barely used, shocked
me by charging a mere 18.00 for a shot an
asthmatic cat needed & yes, we had a full
appointment with the vet himself.

Different vets do things differently. And,
some capitilize on taking your money.

Is there just "a different food to feed the cat"
No. You have to do the work, learning the condition,
the ramifications of being on the diet for "too long"
and researching what other good vets in your area
would do with you....