Q: How long can Julian Assange stay in the Ecuadorean embassy in London?
A: Probably as long as the Ecuador government is happy to host him there. Unless the UK government is prepared to declare all the Ecuadorean diplomats persona non grata, so that the embassy would have to close up and all the Ecuadoreans go home. The cops could then grab him.
If Ecuador could make Julian a citizen of Ecuador, which probably depends on their Constitution, then he could probably go to Heathrow and get on a plane to eg the Galapagos Islands. He might have to give up his Oz nationality, perhaps not too great a cost. International law frowns on nationalities of convenience, however, plural nationality is surely the way of the future.
Julian may be exaggerating the danger of US extradition but he is right to be wary, even if most people accept that as the Monty Python team would say, he's not a terrorist, "he's just a very naughty boy."