Six hours is a school day chunk or a long simmer. You want the alarm, not a memory. Write the end condition on paper too. Software is not the only copy of a six hour plan.
Pick a sound that is obvious. Subtle chimes disappear in a day this long. Press space to start or pause, R to reset, and F for fullscreen. The screen can stay awake after you start, using the wake lock your browser allows. Pick the alarm before you begin. A bell, a chime, a beep, or a short digital pulse all come from the Web Audio API, so there is no sound file to wait on.
This page is the 6 hours countdown and nothing else is running underneath it. Related lengths are linked under the guide if you want a neighbor, and the timer itself stays above the fold.
Questions
How do I run a 6 hours timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. Pick a sound that is obvious. Subtle chimes disappear in a day this long.
What is 6 hours actually good for?
Six hours is a school day chunk or a long simmer. You want the alarm, not a memory.
Will the countdown drift if I switch tabs?
No. The end is a timestamp. The display is the gap between that timestamp and now, so a background tab does not lose seconds the way a ticking counter would.