This page opens with the timer already set to 60 minutes. Press Start or hit the spacebar and it begins; when time is up the alarm rings and the disk blinks. You can always drag the disk to a different time.
Long blocks like 60 minutes tend to sag in the middle. Keeping the remaining time visible makes pacing easy — and the mini timer floats above other windows while you work.
A 1 hour timer is the standard deep-work block. At the halfway point the disk becomes a half moon.