By default, when you leave your iPhone or iPad unlocked, it will lock itself and turn off the display after one minute. This is great if you want to save battery life, but really annoying if you’re using it as a cookbook or other reference that you occasionally glance at. The good news is you can change how long it takes before Auto-Lock kicks in on your iPhone or iPad. Here’s how.
Go to Settings > Displays & Brightness > Auto-Lock.