My cat has just been diagnosed with diabetes, can you please tell me how long he will live?

Hello, my sweet 11-year-old Simba has diabetes, and I am having a hard time giving him his insulin, I am doing the best I can for him, I have even researched a one-time medication for him by mouth, instead of the insulin. Can you please tell me how long cats survive in this condition? If treated properly.

Warm regards,

Carrie Hayes