Well, it depends on the location of the initial outbreak...If it starts in New york, there will be many more infected within 24 hours than if it started say, in Reno, Nevada. Plus, if it started on the east coast, it would spread faster, there's more people per capita in the east vs west. There's too many questions...I'd have to throw up a ballpark figure of two weeks...before the entire world is infested...except Islands...that would be random...hell, there may not even be any sick people on the islands...who knows?
Depending on the gestation period of the virus (i.e how long it takes from showing symptoms to death and reanimation). If the virus had a short kill time, than you wouldn't expect it to travel long distances very quickly. If the virus began in Washington and the virus took 12 hours to kill and turn the victim then Australia would remain uninfected(Flights from America to Australia take 12 hours or so to California). Wise governments would simply close their borders as soon as the virus was identified.
I was thinking about that, the only scenario that fits would be the one from LZ, where the incubation period is months in the planning...that allows people to travel by plane, car and Boat to every corner of the globe...