Evolutionists: How do we determine the length of a half-life?

Well, you could start with 2 K-40 atoms and wait 1.26 billion years, over which time one will decay, OR you could start with 1.26 billion atoms (which isn't very much) and see one decay right away, OR start with 2.52 billion atoms and see 2 decay right away (on average, of course). etc...