Who forty suits
Forty minutes is for people who find that 25 ends just as they get going, but who know an hour is too much on most days. It gives the first ten minutes, spent loading the context, room to pay off, and it still ends before attention usually starts to fade. It is not a measured optimum, just a comfortable middle.
How to use it
Pick one task and write it down before you start. Let the chime end the block even if you are mid-sentence; many writers find a half-finished sentence easier to pick up than a blank page. The break is eight minutes, long enough to stand up and get some water, and after four blocks you get twenty. If forty feels long, try the 30-minute timer; if it feels short, the 45-minute or 50-minute.