For the task you are dreading
Half an hour is a small promise. You are not agreeing to finish, only to work on it until the chime. Most of the dread is in the starting, and once the timer is running it tends to shrink. If it does not, you have lost thirty minutes, not an afternoon.
Half a Pomodoro set
It also fits when you have a gap that is too short for a long block: two 30-minute blocks fill an hour with a break between them. The break here is five minutes, and the next block waits for you to press start.