Ninety minutes is a length we measure whole experiences by: a football match, many a feature film, and one complete sleep cycle. Timing a nap to a full cycle — so you wake at the end of one rather than the middle — is why ninety minutes feels less groggy than a random hour. It is also a classic deep-work block and the length of many exams.
It is long enough that most people run it once, but the editor is there to chain sessions — two rounds is three hours, split in half. Set the count and the 1:30:00 timer repeats in turn.