Two populations of humans were separated for nearly "100,000 years" in Africa. The researchers say they do not know how long it takes for species to fission off into new species, but the genetic material drifted apart before coming back together.
The matter is not of 'how long'. It is the content of change. If certain important genes or gene groupings change in part of a population the split could take only a few generations, or even one generation. These populations form in the way they do and drift as a unit to ensure a reproducing population in an environment, which often takes thousands or more members. If reproductive abilities were widened, the ability for species to genetically drift and evolve would be broadly enhanced and evolution could become much more rapid.