It is possible to test blood glucose for up to 3 hours. The oral glucose tolerance test requires at least 8 hours of fasting before the test, and then fasting venous blood is drawn for fasting blood glucose testing, and then blood is drawn at 0.5 hours, 1 hour, 2 hours, and 3 hours after taking 75 grams of glucose solution within 5 minutes to test the blood glucose concentration at each of these time points. So, it is possible to test blood glucose for 3 hours. However, if you are only testing blood glucose 2 hours after a meal or fasting blood glucose, only 2 venous blood draws are sufficient. People who need to perform blood glucose testing are advised to do so under the supervision of a doctor, and in case of abnormal blood glucose values, it is recommended to go to the hospital.