Feel free to post information relevant to answering What would happen to the ISS if we stopped maintaining it below:
1 week: Mold starts growing unabated - Without the crew's weekly cleaning sessions, mold is allowed to grow on interior surfaces.
Within ~1 year: Some space junk collisions have occurred - Without active ground monitoring, the station is no longer able to perform collision avoidance maneuvers. A collision worth avoiding may not occur for more than a year or two, but is certainly possible within a year.
Within ~2 years: Rust starts to form on some interior surfaces.
My own concerns:
How long would the solar panels stay rotating before the bearings grind to a halt
Would the air eventually leak
Would the temperature remain stable
As we are told many times, the value of stuff in space is not the stuff, it's the "in space". There would be a huge economic pressure for another country or party to go up there and (at least try to) claim it as abandoned property for salvage. I can't say it would happen, but there would be several parties that would give it some serious thought.