I don't believe there is such a thing as generic willpower. A person has far more ability to do something when are reminded to do it.
And I've experimented with timers in the computer and I've found they don't work as well. The egg-timer gives me a full "UI" without consuming desktop space - I can stretch early and reset the timer easily without, say, returning to the machine itself. There's no conflict with system sounds, etc.. A thing outside the computer is more suited to getting me ... outside the computer.
And egg timers/kitchen timers are pretty cheap - if it doesn't work, you use it to time your eggs...
$ while true; do sleep 20m; xmessage 'stretch' &; done