This question came up in the Online 10-day Retreat Nov 3, 2020, guided by Delson Armstrong. Day 3 was part of a daily 30 -minute discussion on the suttas, the Dhamma talk and reflections.
Slightly edited to improve readability
For one, if you jump from one and a half hour to two hours, it’s quite a jump for the mind to get used to. So, you do it in increments of five to ten minutes. So, an hour thirty-five minutes, an hour forty minutes. That works better than jumping from one to another like half hour scales. It’s better to do in those more manageable ways. You might find it easier for your mind to say: ok, one hour thirty-five minutes, rather than jumping from one and a half hour to two hours straight.