How long should a time block be?
Most time blocking advice arrives with a number attached. Ninety minutes, because of ultradian rhythms. Two hours, because deep work needs runway. Twenty-five, because of the pomodoro.
All of these are someone else’s number. The useful one is the longest block you reliably finish, and you find it by measuring rather than by reading.
Why too long is worse than too short
An over-long block does not fail neutrally. It fails expensively.
You lose attention at minute forty of a two-hour block. Now there is eighty minutes of block left, visibly unfinished, and you are aware of it. Most people respond by abandoning the block entirely, then the afternoon, and on a bad week the plan.
A thirty-minute block that goes well ends in success and you start another one. A two-hour block that goes well for forty minutes ends in failure, despite containing exactly the same forty good minutes.
The asymmetry is the point. Undershooting costs you a little efficiency. Overshooting costs you the plan.
Finding your number
Two weeks, no special tooling.
- Pick a starting length. If you do not know, start at 30 minutes.
- Give every block a planned length and record what actually happened: finished, ran over, or abandoned.
- At the end of two weeks, look at the completion rate by length.
You are looking for the longest length where you finish most of the time. Not all of the time — a plan you always complete is a plan that is too easy. Somewhere around three in four is a good target.
Most people who have struggled with time blocking discover their number is well below what they were attempting. Twenty-five to forty-five minutes is common. This is not a deficiency; it is just information you did not have.
Any tool that compares planned against actual will show you this. Focusly’s analytics breaks completion down by block length specifically, which shortens the two weeks to a glance, but a notebook and a tally works.
Adjust by task, not just by person
Your number is not one number.
Familiar, mechanical work — email, admin, code review — usually supports longer blocks. There is no starting cost and no ambiguity.
Ambiguous or creative work — writing, design, anything where you do not know the shape of it — usually wants shorter blocks, because that is where attention wanders and where the “what am I even doing” moment lives.
Work you are avoiding wants the shortest block you can justify. Fifteen minutes. The goal is starting, not progress.
A useful habit is to write the length into the plan deliberately rather than defaulting to an hour because an hour is a round number.
On running over
Running over is not failure, as long as it happens because things are going well.
The distinction worth drawing: a block that ends and you choose to continue is a success. A block that ends and you do not notice for forty minutes is a different thing, and it usually means the end of the block was not marked clearly enough — a notification you can dismiss without reading, or no notification at all.
Something should make a moment out of the ending. Then continuing is a decision rather than a drift.
The short version
Start at thirty minutes. Record what you finish for two weeks. Move the number up only when you are finishing three quarters of them. Use shorter blocks for anything ambiguous, and the shortest you can bear for anything you are avoiding.
The correct block length is not the one that would be most productive if you sustained it. It is the one you actually finish.