I also use a it this way, but instead I do 90/30 (work/break) as 90 minutes gives me enough time to finish most exercises and tests without having to hurry.

I do around 4 to 5 sessions per day which hits the sweet spot for me as I can endure this for multiple months without having to take days off. This comes at the expense of having to start a few weeks earlier to have enough time to prepare for tests compared to cramming 12 hour days the week before