Why 8 minutes
Eight minutes is a firm-boiled egg with a just-set yolk, and a common length for a warm-up before a run. It is also roughly how long light takes to reach us from the sun, which is not useful but is the kind of thing you think about while waiting for a timer.
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 8 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.