So, when do I know that it is OK to take the dry food away? How will I know he is getting enough food?
It's pretty much all about how many calories he consumes. The "calories per can" data is on Dr. Lisa's food chart to the far right.
When a cat is at an ideal weight, without diabetes, it takes about 15-20 calories per pound of ideal body weight in order to maintain that weight. Every cat is different, and some of it has to do with how active they are and how fast or slowly they burn the calories.
But with a diabetic cat, especially at first, they need more calories than "normal" because they don't metabolize the food at 100% efficiency.
So, what does Coffee weigh now? And is he overweight, underweight? That can at least give you a ballpark number of calories he might need per day.
Pre-diabetes, Bob weighed 22 pounds. Quite the little porker

The day he was diagnosed, he was 12 pounds. I thought he was looking pretty good but apparently he wasn't doing well at all. My goal, per my vet, once he started treatment was to get him back up to 14 pounds. When he was on insulin, I tried to feed him about 250 calories a day. It worked, and he added 2 to 2.5 pounds. Since then, he's maintained that weight on 250 calories or so per day.