Apple - How can I extend the lock screen timeout in iOS?
Without jailbreaking your iPhone there Is no option to change that.
Everything you need to know (I think) : First, the OP is about whether it’s possible to extend the amount of time the lock screen stays lit, not how long it takes before the unlocked device’s lock screen kicks in.
In doing some experimentation with an IPhone 6 and IPad Pro (2nd gen) both using iOS 12.4, Apple has set 2 timers for the lock screen and 1 timer for the widget screen (accessed by swiping to the right). None of the timers are adjustable in Settings
Without a notification present, the main lock screen will remain lit for 5 seconds before going black. With a notification present, the lock screen remains lit for 10 seconds before going black.
The Widget screen will stay lit for 10 seconds before dimming and then 5 seconds later will go black.
Things you CAN’T do to keep the lock screen lit:
use the sleep/lock button. This does nothing but turn off the lock screen prematurely. Once the lock screen goes black, pressing it again will bring back the lock screen and reset the 5 or 10 second timer. As suggested in other posts, there doesn’t appear to be any tricks to use the sleep/lock button to increase the length of the timer.
Pressing the Home button. Pressing it brings up the login screen. Once the lock screen goes black, pressing it will bring back the lock screen and reset the 5 or 10 second timer.
Things you CAN do to keep the lock screen lit:
pressing one of the volume buttons (up or down) every 3-4 seconds will reset the timer and keep the login screen lit.
keeping a finger on the screen within the notification display area (near or on the actual notifications) AND moving the screen slightly up and down or physically sliding your finger side-to-side every few seconds will keep the screen lit.
keeping a finger on the screen near the left or right edges of the lighted screen AND moving the screen slightly side-to-side every few seconds will keep the screen lit. Physically moving your finger up and down or tapping the screen in this area will not work.
Bottom line : the best solutions to keep the screen lit in order to read notifications appear to be to press the volume buttons or use a finger near the left or right edge of the screen as described above. For me personally, placing my right thumb in the bottom right part of the screen and slightly moving the screen side-to-side keeps the screen lit while I use my left thumb to scroll through the notifications.
Hope this helps:)