Why 50 minutes
Fifty minutes is the academic hour — a scheduled hour with ten minutes taken out to get to the next room. It is also the length of a standard therapy session, for the same reason: the clinician needs the remaining ten minutes to write notes before the next person arrives.
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 50 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.