Past the minute
A hundred seconds is roughly the point where every burst method has already failed. Jitter is gone by forty seconds, butterfly by sixty for most people, and what remains is whatever your hand does when it is no longer trying to be clever.
Readings here are lower than anywhere else on the bench and steadier than anywhere else. A hundred-second result repeated a week later will normally land within twenty CPM of the first — which makes it the best interval for tracking whether practice is doing anything.