How many days did it take to ordain Aaron and his sons as priests?

Answer: Seven

Moses told them not to leave the entrance of the Tent of Meeting for seven days, "for it will take seven days to ordain you."

“You must not go outside the entrance to the Tent of Meeting for seven days, until the days of your ordination are complete; for it will take seven days to ordain you.”— Leviticus 8:33 (BSB)