Since then the ISS crew size has been expanded. There have been subsequent Soyuz port relocations (and will be future port relocations of Crew Dragon and Starliner) but none have left/will leave the station completely unoccupied. So the "true" start of permanent occupation of ISS was 9/27/2007.

When was the ISS unoccupied?

The International Space Station (ISS) NASA launched its first space station, Skylab, in 1973. Three crews were sent to live and work there in 1973-1974. It remained in orbit, unoccupied, until it reentered Earth's atmosphere in July 1979, disintegrating over Australia and the Indian Ocean.

How long does the ISS have left?

Each ISS module has a planned lifetime of 10 years; by that reckoning, the entire space station will need replacing by 2020. That said, the ISS has an ongoing programme of maintenance and renewal – so I reckon it should keep going for at least another 15 to 20 years.