For how long i should keep the tacrolimus ointment0.03% on? my doc suggested me to use it twice a day but didnt tell me how long should it be there on the affected area. i've white patches near my lips?

Products applied to the skin surface are put in place and left there until they wear off or are lost from body sweat. You do not time their application & attempt to remove them. A thin layer of an ointment, applied thick enough to know it is there but not thick enough to blot off easily is the right amount. Repeating 2X/d assumes their loss through the day.