Twenty minutes is the round, friendly block the day keeps coming back to: a complete short workout, a guided meditation or yoga sit, a focused reading or study stint, a capped slot of screen time for the kids, and the upper edge of a power nap before deep sleep makes you groggy. Long enough to finish something real, short enough that you will not keep putting it off.
Most people run twenty minutes once, but the editor will chain it — three rounds is an hour, split into thirds — for consecutive blocks you do not want to reset between. When it is a workout you are timing, though, a clock that counts the rounds usually beats a flat repeat.
An 18 minute timer — the length of a TED talk, and a tidy focus sprint. Repeats as many times as you set.
A 24 minute timer — about a sitcom with the ads cut out, or one long focus block. Repeats as many times as you set.