In settings you should be able to set the sleep cycle: Settings/(Device) Display/Sleep.
Though it would be nice for your reading app to interrupt this setting, a la Netflix. Where are you having this problem?
I am testing Kindle App, Google Books, and Pocket to see if they respect the device setting or ignore it for a better reading experience. I'll edit as necessary. My device is set to sleep in a minute.
- Kindle App ignores it. I'm not sure of the timeout, but it has remained alive for far more than three minutes so far, sitting on an open page unattended.
- Pocket went to sleep in a minute, per my OS settings.
- Google Books ignores the setting. It has been sitting on an open page for more than four minutes.
Now I am interested in finding the ceilings of these programs.