Answer:
Several generations
Step-by-step explanation:
For a completely new species to branch out of its predecessor, a series of germline mutations are needed. Each mutation is spontaneous , random, directionless and depends on individual and environmental factors. Only favourable mutations (purely on luck) pave the way to speciation.
Thus, speciation is not a single generation phenomenon. It takes several generations to fully manifest. The phenomenon of mutation being a random event makes speciation an ever longer procedure.