As long as you want to!
Seriously--scabies stay on the body until they are treated, causing itch which is more and more maddening, until sleep becomes impossible. This takes months.
If you treat scabies, the itch goes away in 7-10 days, but the scabies mites are dead by the next morning. Mites can only live off the body for an hour or so.
If you have been treated for scabies and you aren't better in 2 weeks, then the diagnosis needs to be reconsidered.
Best.
Dr. Rockoff