A 55-year-old male, height 172cm, weight 80kg, presents with increased appetite, easy hunger, accompanied by palpitations and hyperhidrosis for 2 months. Physical exam: marked obesity, irritable mood, slightly moist skin, thyroid gland not enlarged, heart rate 108 bpm, BP 140/70 mmHg, no fine tremor in both hands. Current HbA1c is 8.7%. How long has the patient's blood glucose level been persistently elevated?