Is it possible to set how long the lock screen will disappear on the iPhone?
There is no setting to customise this. The only solution would be to do a screen action so it stays on for longer. Like, swipe up to see your notifications or press the Home Button to the passcode screen and return to the wallpaper screen.

On iOS 12 there is a solution. Unlock your phone, swipe down from the top and view your notifications from there. On this screen the iPhone will dim according to your Autolock settings which can be changed in Settings -> Display &amp; Brightness -> Autolock.