What's neat is that this is a differential equation. If you kill 5% of instances each hour, the reduction in bad instances is proportional to the current number of instances.

i.e.

if bad(t) = fraction of bad instances at time t

and

bad(0) = 0

then

d(bad(t))/dt = -0.05 * bad(t) + 0.01 * (1 - bad(t))

so

bad(t) = 0.166667 - 0.166667 e^(-0.06 t)

Which looks a mighty lot like the graph of bad instances in the blog post.