Your doctor may recommend a certain time period for fasting; however, the standard amount of time is about eight to 12 hours of no food before having blood drawn. Schedule your blood test for early in the morning so you'll only need to fast overnight to meet requirements. Fast for eight hours before a glucose test. The test is typically conducted in the morning while your body is still in a resting place for a more accurate reading. You must fast at least 12 hours before taking a cholesterol blood test to get an accurate reading on triglycerides. The American Heart Association warns against quick cholesterol tests you find in malls and health fairs because fasting is imperative to obtaining an accurate result.