Answer: When blood glucose levels rise beyond the normal level (hyperglycaemia), an insulin injection is needed.
Explanation: This applies to insulin-dependent diabetes, i.e. Type I, where the medical diagnosis is insulin insufficiency. The situation with Type II diabetes is less straightforward.
Insulin is required to enable glucose to be distributed as required, taken up by cells as they need it. Glucose is constantly produced. Without insulin, little glucose is removed from the blood, and the blood glucose concentration rises. The symptoms of this hyperglycaemia include unusual thirst, frequent urination and tiredness. Unless very high levels are reached this may not be severe enough to be noticed, which is why diabetics monitor their blood glucose levels - to avoid those severe episodes. If their blood glucose exceeds normal levels, an insulin injection is needed