As others (and yourself) have said, disturbances in spacetime move at the speed of light. That includes the returning of spacetime to it's initial position. However, that's not the complete story, because in a blackhole, the masses are large enough to cause spacetime to jiggle. But then your question is something like "how long does it take for a pendulum to stop swinging?"
This question is not, in general, specific enough to answer. But in the famous LIGO chirp, the gravitational waves that emanated from the jiggling took about 0.1 seconds to settle down to the point where we could no longer detect them. This conflates multiple ideas such as the sensitivity of our instruments, the size of the colliding blackholes, and the distance between us and them.
This was an event that was somewhat extreme though. For a (very) brief moment it was brighter than all the stars in the universe.
So, to answer your question: in general it never returns but continues to jiggle forever. But in almost all circumstances and with the measurable precision that we have, it returns immediately (speed of light). Somewhat similar to the length of a piece of string