type-break-keystroke-warning-intervals

type-break-keystroke-warning-intervals is a variable defined in `type-break.el'.
Its value is

(300 200 100 50)

Documentation:
List of keystroke measurements for warnings about upcoming typing break.
At each of the intervals (specified in keystrokes) away from the upper
keystroke threshold, print a warning in the echo area.
If either this variable or the upper threshold is set, then no warnings
will occur.

You can customize this variable.