Although Settings/Display allows you to set the time the screen will stay on, many Android devices suffer from a flaw, to-wit: This setting continually reverts to 1-minute. It is a known, pervasive, and ubiquitous issue with no apparent device-internal fix. There are many apps that address the issue but, unfortunately, most do so in a sideways or orthogonal fashion (e.g., requiring you to select each app you want the stay-on setting to apply to). Fortunately, there is one simple, free app that gives you three options: use system settings (which is kind of superfluous given that you are using this app because the system settings don't work); stay on until you turn off the device; or stay on for X minutes (where you choose the value for the variable X). It's called Keep Screen on Free.
Link to Keep Screen on Free in Play