Why 7 minutes
Seven minutes is the length of the well-known seven-minute workout — twelve bodyweight exercises, thirty seconds each, ten seconds between — published in the American College of Sports Medicine's Health and Fitness Journal in 2013. It became popular precisely because seven minutes is hard to argue you do not have.
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 7 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.