This page opens with the timer already set to 90 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 90 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.
90 minutes matches one natural focus cycle (the ultradian rhythm) and many standardized-test sections.