Ignoring the tails humans already have.
Let's say it's the year 2016. Merpeople exist. They live in the ocean or rivers, breathe air, and have to surface occasionally. They also evolved from earlier humans.
They're pretty diverse, maybe as far as having different subspecies of merfolk.
Under intense selective pressure, how long ago would merpeople have to diverge from landpeople to develop tails, from legs, fit for swimming?
Side question: what would they look like compared to fantasy version of mermaids?
Edit: Being in the genus "homo" counts as human. Merpeople do not necessarily need to be in this genus at the "end" of their evolution, but must for the start