Why 25 minutes
Twenty-five minutes is one Pomodoro, the interval Francesco Cirillo settled on in the late 1980s using a tomato-shaped kitchen timer. The number is not magic and Cirillo has said so — the value is in the boundary, not the length — but it has become the default that every focus app ships with.
It will not drift while you do something else
The countdown is worked out from a fixed end time rather than counted down frame by frame, so putting the tab in the background, locking your phone, or switching apps does not cost you any of the 25 minutes. Browsers throttle background JavaScript to save battery, and that is exactly what makes most browser timers lose time. Press start, go and do the thing you are timing.