I’ll refer to type II diabetes, called diabetes mellitus (sugar diabetes). You will not find a clean, definitive answer to the question in the title because it is a poorly formulated question.
Type II diabetes can be managed. With dietary management and daily injections of insulin you can regulate most diabetic cats enabling them to live normal lives.
Conversely, in advanced cases of feline diabetes there is loss of appetite, vomiting, weakness, acetone breath, dehydration, lethargy and coma.
About 70% if diabetic cats will require at least some insulin. Many cats enter a phase when the diabetes corrects itself when there is no need for insulin.
There is a type of diabetes called ‘transient diabetes’. These are type II diabetics requiring insulin initially but over time they don’t need it; moreso if their diet is improved to a high-protein, low carbohydrate diet.
In the human world it has been shown in a recent study that weight loss can literally cure diabetes in a substantial percentage of diabetics depending on the amount of weight lost. I see no reason why this does not apply to cats. One veterinarian says that if you take a diabetic cat off dry cat food and onto high quality wet he’ll lose weight and cure his diabetes. This is a change to a low carb. high protein diet.
What I am getting at is that if the diabetic cat’s owner commits to excellent management of diet and insulin administration for as long as it is required the life expectancy of their cat should be normal (around 18 years today on average for a random bred cat).
However some diabetic cats, because of circumstance, live shorter lives. If, then, we are to take an average life expectancy we have to factor in these shortened lives. The average will drop. I’ll have to guess by how much because there are no published figures on this.
My guess is that the average life expectancy of all diabetic cats will be a few years shorter than the normal, say around 13-14 years. This is strictly an average.