Two minutes at the gym, then everywhere else
Two minutes is a common rest window between sets. It is long enough to let your breathing settle and short enough to keep the session moving. You can start the countdown, step away from the screen, and still hear the alarm from across the room. The tab title shows the time left, so you can check it from another window.
The same length works for brushing your teeth, which most dentists suggest for two minutes. It also fits a quick tea steep when you want a lighter cup, a timed reading sprint, or one short section of an exam. A two minute presentation slot is another common use. The timer does not care what you are doing. It just counts down from 2:00 and sounds a tone when it reaches zero.
This timer reads the computer's clock instead of counting ticks. That means it keeps honest time when the tab is in the background or the device sleeps. The display refreshes several times a second. A running timer also survives a page reload on the same page, because the end time is stored in the browser for that tab.