Cold shower timer

One minute is a reasonable place to start, and long enough to be unpleasant, which appears to be most of the point.

Be careful with the claims. The research on deliberate cold exposure is genuinely mixed, and the strongest consistent finding is that cold immersion after resistance training may blunt some of the muscle adaptation you were training for. If you lift, the timing relative to your session matters more than the duration.

The alertness effect is real and immediate, and it does not need a stopwatch to notice. A minute is plenty.

Or set any length you like: gotimer.app