There's a fairly short answer: quite quickly. It depends on what you define as 'rock and roll' of course; genres are as much about culture as they are about the particular sounds made, because the sounds made that are deemed as important have to be deemed important by someone, and different someones have different definitions.

So a variety of different songs get called 'the first rock'n'roll song': perhaps the most common is 'Rocket 88' by Jackie Brenston (but really Ike Turner's Kings Of Rhythm, a group from Mississippi) in 1951, and Bill Haley's cover of that song three months later in the year. But such songs as 'Rocket 88' don't really feel like a 'regional phenomenon'; at the time, they were just part of the wider genre of rhythm & blues, and it's only in hindsight that we put more stock in those songs, primarily because of what came later. And basically from 1951 Alan Freed was broadcasting a white-teen oriented radio show playing a mix of upbeat tunes that would now feel a little bit rock'n'roll, and which Freed described as such - in Cleveland, Ohio and then in New York City, neither of which are part of the South. So even in the pre-history of rock'n'roll, there was clear participation by people not from the South.

Probably the first time you can see something that's fairly uncontroversially rock and roll being a specifically/clearly Southern regional phenomenon is with the Sun Records recordings of Elvis Presley. The first Elvis track recorded for Sun Records was in 1954 ('That's All Right'), and Elvis did very quickly become something of a phenomenon in the South over the next year, prominently performing on television and radio shows and selling a sizeable amount of records for an independent Southern label like Sun. Sun Records' Sam Phillips sold off Elvis's contract to a (Northern based) major record label, RCA Records, in November 1955.

The reason why Elvis's contract was gobbled up by a major record label was of course, the worldwide success of Bill Haley's 'Rock Around The Clock' in mid-1955. Elvis was clearly already a phenomenon within the South - in a way that, say, Ike Turner never was, thought 'Rocket 88' was certainly an R&B hit at the time - by the time 'Rock Around The Clock' hit it big, but I suspect Elvis's association with the new style of music that was at the top of the charts didn't hurt his commercial prospects in the South, even if Bill Haley was from Pennsylvania. But of course, by the point that 'Rock Around The Clock' hit the charts in a big way, Bill Haley was himself, in his own way, something of a regional phenomenon, just in different areas of the U.S.; the cover of 'Rocket 88' I mentioned before was a regional hit. 'Rock Around The Clock' is quite different in a variety of ways to 'Mystery Train' or 'That's All Right' - Bill Haley's music has a much bigger beat than Sun Records Elvis, and is more obviously influenced by the musical style of Western Swing (which Bill Haley's music originally was).

That said, very many of the early crop of rock'n'rollers in 1956 and 1957 were from the South: Chuck Berry was originally from St. Louis (though was recording for Chess in Chicago). Jerry Lee Lewis and Carl Perkins' careers in 1956-1957 were initially funded by Sun Records selling Elvis's contract. Little Richard was famously the Georgia Peach. Both the Everly Brothers and Buddy Holly was from Texas. Fats Domino (who had been having regional hits from 1949) was from New Orleans. Probably the biggest names in rock'n'roll from the North, apart from Haley, in the 1950s, were The Drifters and The Coasters, who were both essentially based around New York (where Atlantic Records and their producers Leiber and Stoller were).

Of course, the primary influences on rock'n'roll are music forms that are ultimately from the South; what's unique about 'Rocket 88' amongst the rhythm & blues of its era is that Ike Turner & co brought a very Delta sensibility to the upbeat jump blues style. What's unique about Elvis's Sun Records recordings is that Elvis simultaneously sounds like a country singer and a rhythm & blues singer. Bill Haley is in a lot of ways what it sounds like when a Western Swing band tries to play rhythm & blues. Chuck Berry very deliberately tried to make a form of rhythm & blues palatable to white kids, and part of his formula was a dollop of country/folk ('Maybellene' famously having a big influence from the folk song 'Ida Red').

The rhythm & blues of the early 1950s era that was the mother of rock'n'roll is very much characterised by a melting pot of ideas, with the classic example being someone like Muddy Waters, who grew up in the Delta, was recorded playing his songs on an acoustic guitar, delta blues style, by a song collector. Waters moved to Chicago with the Great Migration, where he found a new style playing electric guitar with a small band, and rose to prominence in the late 1940s. Mostly, what rock'n'roll did was - knowingly or not - isolate the parts of rhythm & blues that were the most Southern, add a little bit of country/folk, and then, basically, speed it up and play it loud. In terms of the musicians making it, it's probably easier to get that mix right, to get the vibe right, when you grew up with it, which is perhaps why Southern musicians like Elvis or Little Richard tended to do it very well. For white teenagers of the era, something about this mix of sounds and the way of playing it expressed a certain desire to loosen the shackles - but there doesn't seem to have been much of a delay between Southern kids getting it and the rest of America - or indeed the world - getting it.