Where the default numbers come from
The defaults follow the Pomodoro Technique as Francesco Cirillo described it in his 2006 paper: 25-minute work intervals, a 3 to 5 minute break after each, and a longer break of 15 to 30 minutes after every four. Cirillo also wrote that, in the experience of the people he taught, a work interval of 20 to 35 minutes suited most people, and 40 at most.
Change the numbers to suit the task. Direct research on the technique itself is limited, and the studies that exist do not settle a best length. The guide to the Pomodoro technique for students describes what those studies did and did not find.
Tip: Decide what each block is for before you start it, such as "practice questions 1 to 6", so the timer ends with something you can check.