Which best describes how long it usually takes for speciation to occur? several generations one generation several years several days

Several generations

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.