Assuming the diagnosis was correct and something else isn't going on, you
should have seen improvement rather quickly, a few days at most to reduce
or eliminate the primary symptoms (extreme hunger/thirst/urination) and in
another month or two the muscle wasting/weight loss should have stopped or
perhaps even been turned around.

What does your vet say? Something else might be going on, such as a kidney
problem, assuming the symtom you still still see is extreme urination.

Be sure you're giving the right amount of insulin. Double check that you
have the correct match up between insulin type, syringe calibration, and
the numerical amount you were told to give. Make the vet or pharmacist
verify all these specifics for you. (You're not shaking the insulin, are
you? To mix you roll the bottle gently; do not shake. Store under
refridgeration)

Is the cat now on a food formulated for diabetic cats? (In less extreme
cases that alone can go a long way to correct the problem.)

Definitely check with your vet, or perhaps find a new vet if your current
one doesn't seem very helpful. (My old fellow, now 18, has been diabetic
for 6 years -- two shots a day. He's doing quite well, considering the age
and the disease.)

Frank