400 aircraft at, lets say , 1 per minute, is near enough 6 hours. (They are microlights, they can land quickly). Sandown opens at 08:00 so aircraft No 400 wil rumble t o a halt at 02:00.
But Sandown closes at 1700, so if all 400 aircraft want to leave, a/c no1 will be departing at 11:00.
Its going to be a bit busy chaps!