I aim at
30 minutes - 1 hour
Unfortunately, it might 15-20 minutes sometimes
There were two time periods in my life when I could read through the night.
Age 14-16.5, the last two year in the High school. I would use dad's mining headlamp under the blanket.
Then, much later, age 23-27 when I was working shifts in a factory. Sometimes when I had an evening shift that officially ended at 23.00, with a couple of overtime hours, I would come home at about 01.00-02.00 and read up till 06.00

Wake up at 13.00 for the next shift, so it was not a problem

Not anymore though. I have to think about the next day and my stern sentry won't let me anyway. The longest I stayed on weekday was 01.00