Planes get C-checked approximately every 15–21 months. This check puts the aircraft out of service and until it is completed, the aircraft must not leave the maintenance site. The time needed to complete such a check is generally 1-2 weeks.
The night after the planes go to C-check, they are washed. The main purpose of the wash isn’t cosmetic. It’s to clean up components for inspectors the next night to inspect everything and get a good look.