What is the blood oxygen saturation levels are supposed to be after tetralogy of fallot?

Low. Pts with tetrology of fallot have shunting of blood so they will have low oxygen saturations.
Depends on Anatomy. This is very dependent on the specific anatomy of the patient, and the specifics of the surgical technique. There are many factors (heart, lungs, chamber pressures, etc) that can affect the oxygen level after tetralogy of fallot repair, and each situation must be evaluated individually. In the long run, however, the ultimate goal is normal saturation levels (98-100%).
Normal. If by "after tetralogy of fallot" you mean after repair, oxygen saturations should be normal after recovery from surgery.