I need to take my annual blood test. How many hours do I need to fast?

A range of time. As different labs will request between an 8 to 12 hour fasting prior to phlebotomy.
Usually about 10-12. One can usually fast after 9:00 p.m. at night, and go to the lab around 8:00 a.m. in the morning. The fasting is for getting more useful blood glucose and cholesterol numbers.