Caff

Caff provides a quick settings tile that allows the display to be kept on for an extended period of time.
By tapping the tile, it is possible to toggle caffeine mode and cycle between different preset durations. The tile also serves as a countdown timer when caffeine is enabled.
By long-pressing it, a preferences menu is shown, allowing the user to choose which durations will be cycled through.
Additionally, there is an "infinite" mode, which will keep the display on indefinitely.
Note

You may have to disable battery optimizations for the app to work.