I can tell ya when it's complete, when ATC says so, that's when.
Proof:
Late night (1am), new pilot making first night landing with crosswinds, cleared ILS approach, in weather, at a field with a closed tower, so I didn't think about the coming complication. Break out below radio coversge, vacate the runway, do the "after landing", thank the pax, help get new plane put "to bed" with ground crew who are unfamiliar with new plane--lots of "stuff" happening. Wait, you know what's next....."Sir, the Fire Department called, ATC wants your flight plan closed."
So, the approach is over when ATC says so.
GF