Every person on Earth, I’m told, is probably at least 50th cousins with every other person.
Similarly, because we and the great apes share a common ancestor, theoretically I could send a time machine back through the generations, grandfather before grandfather back, and find the primate that I and the chimp at the zoo share as a common ancestor.
Not that I’m planning on a bigger rental hall for the next Bricker Family Reunion, but what order of magnitude of cousin is that likely to be? How about farther outside species? Consider a crocodile, unchanged as a species for millions of years. It’s still the case that if you roll back the clock far enough, we probably reach a creature whose line of descendants branched out into what would become crocodiles and what would become primates.
What order of magnitude, then, of cousin would a crocodile be to me? How about a shark? (No lawyer jokes here, please…)
The branching between the line of early amniotes (earliest “reptiles” in the loose sense) that led to the mammals (synapsids) and the one that led to other “reptiles” including crocodiles and birds (Reptilia in the narrow sense) happened a very long time ago. The earliest synapsids split off from other amniotes more than 320 million years ago.
Many of the ancestral forms of both humans and crocodiles were small, and may have had a generation time of a year or less. The more recent ancestors may have had generation times of a decade or more. Offhand, I would estimate that there have been at least 100-200 million generations between humans and crocodiles.
It is less certain when the ancestors of sharks diverged from other vertebrates, but it was at least 418 million years ago and probably more than 455 million years ago, in the late Ordivician Period. So lets say at least 200-300 million generations for them.