Hi Lucy
You might find the "second hand" engine was more likely a "spare" engine, something that costs millions and millions to have sitting around waiting for the need to be used. Given that the aircraft returned to its main base i would suggest 24hrs would be the norm, but it depends on the nature of the failure and conments already made.
These things happen, my airline has an aircraft sitting on the ground after a bird strike, 48hrs. We'd have preferred it be ten minutes but life is never so easy. Safety always comes first.
