Any time of day is fine, but you should take it at the same time each day.
According to the manufacturer, Pristiq is a long-acting medication with a half-life of approximately 11 hours. After 4-5 days, you will have a steady amount of medication in your bloodstream (same amount of medicine in your body all day long).
Taking it at the same time each day is important. Also important: if you go off Pristiq, you need to do so gradually.