Eighteen minutes is the famous cap on a TED talk — long enough to make a real point, short enough to hold attention, which its organizers chose deliberately. That makes it a natural limit for rehearsing a talk, a focused work sprint, or any task you want to keep tight and finish before attention fades.
Two rounds of eighteen minutes, with a break of your own between, is a thirty-six-minute work block. Set a round count in the editor to repeat it — though for focused work, a timer that schedules the breaks for you is often easier.
A 4 minute 30 second timer — 4:30, roughly a song or a French press brew. 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.