As pointed out by Sasquatch a single 'critical' mutation could accomplish the reproductive barrier immediately - but first it would have to rise to fixation within the population (become the dominant allele or genomic form). This means it would have to confer some strong survival or reproductive advantage.
A quick clarification: In the case of chromosomal rearrangment mutations, fixation doesn't require the same sort of strong fitness advantage to reach fixation. Instead, you've basically split one chromosome set into two types of chromosome sets; BUT, they both contain the same genome/genetic information.
So it's all a matter of segregation - generally, offspring who are heterozygous for the original and rearranged chromosomes generally do not survive or have severe reduction in fitness. Thus, if some offspring happen to get a complete set of the 'new' chromosomes, they may look identical and have the same fitness level as other in the population with the original chromosome set, but they are now reproductively incompatible because their genome is incapable of combining to form a genome that will segregate properly during cell division.
So 'fixation' is a more a matter of luck, and is likely to happen rather quickly or not at all.
I'm not sure that I've explained this well, and it is a bit complicated with bits of chromosomes jumping about - it may be helpful to search out "Robertsonian translocations" on the web or pubmed if you are interested in the subject. "Robertsonian translocations" are such rearrangments in mice, and have been fairly well-studied, including at the level of their role in speciation.