Since Cambrian and later rocks are largely sedimentary and igneous (volcanic) rocks are found in Cambrian and later strata, if these rocks are really 550 million years old, then life must also be at least 550 million years old.
In this video I walk you through using the simulation.
For potassium 40, the half-life is about 1.3 billion years.
In general, in one half-life, half of the parent will have decayed.
We can assume that the Precambrian rocks already existed when life began, and so the ages of the Precambrian rocks are not necessarily related to the question of how long life has existed on earth.
The Cambrian period is conventionally assumed to have begun about 550 million years ago.