How long was Miriam confined outside the camp after she became leprous?

Answer: Seven days

Miriam was confined outside the camp for seven days, and the people did not move on until she was brought back.

“But the LORD answered Moses, “If her father had but spit in her face, would she not have been in disgrace for seven days? Let her be confined outside the camp for seven days; after that she may be brought back in.””— Numbers 12:14 (BSB)