This page opens with the timer already set to 30 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.
30 minutes is a great length for single-tasking. Watching the disk shrink creates a gentle deadline effect, so the same minutes feel more productive.
A 30 minute timer is a comfortable unit for reading, workouts, or a burst of house chores.