Speciation is really a judgment call. We don't really have objective criteria that says "99% or more genetic similarity is the same species".
But that assumes that there is evolution happening in the first place. Plenty of organisms are quite happily living in the same form as they did hundreds of millions of years ago. The nautilus, for example, evolved about 500 mya, and remains largely unchanged today (though many of its siblings are extinct, and the nautilus itself is endangered). For simpler organisms, you can probably find examples much older.
Edit: forgot to answer your question directly. It could be "never".
If I could add, it's likely impossible to say, because evolution is driven by selection pressures.
If the original strain AA has descendent strains AA, AB, and AC, we can't know with any certainty which is more fit to survive, because it could be one, two, or all of them simultaneously.