It can be left unattended indefinitely as long as astronauts can at least get up there occasionally to perform maintenance. Without that minimal level of maintenance, though, the ISS runs a serious risk of becoming uninhabitable. And, of course, all the experiments and stuff currently on-board would be lost.
Remember, the Russians still can fly crews to the space station.
True, but how long will ISS be viable without substantial US participation. If we have to pay Russians to do that which we intended to do then the program will crumble. There has been an incredible anount of friction between the USA and Russia during the execution of this project. I just can't see a way out of this box which would be palatable to NASA and Congress. Of course, I might be biased. I remember when the Space Exploration Program lost out to ISS by one vote, in 1992 I think. I was employed by the Space Exploration Office and decried the circular reasoning that allowed the justification of ISS and shuttle.