How to set custom lock screen time in Ubuntu 18.04

Open Terminal and run:

gsettings set org.gnome.desktop.session idle-delay 1800

to set the "Blank screen" delay to 30 minutes or 1800 seconds (or set any value in seconds). You can use $((30*60)) to use minutes directly.

Note: After making the change, in Settings → Power → Power Saving → Blank screen it will show "Never" as there is no entry for 30 minutes.

To lock: The number of seconds after blank screen activation before locking the screen (default: 0)

gsettings set org.gnome.desktop.screensaver lock-delay 0

Needed: Set this to TRUE (default) to lock the screen when the blank screen goes active

gsettings set org.gnome.desktop.screensaver lock-enabled true

The timeout for locking the screen after it goes blank can be achieved via GUI.

When using Ubuntu 18.04 open Settings. There select the Privacy settings tab. Now the Screen Lock option will be visible. When clicked, a modal window with the specific settings will open.

There you can toggle Automatic Screen Lock on/off, set a time for locking the screen or respectively binding it to the screen turning off via Lock screen after blank for, last but not least Show Notifications lets you decide if notifications should be shown on the lock screen.

So the path for the option you want to change is:
Settings → Privacy → Screen Lock → Lock screen after blank for
There you can select 30 minutes as the lock screen time.

There are 2 different settings, one for blank screen and second for locking screen after blank.

1. Set timeout of "Blank Screen"

• In GUI: Settings → Power → Power Saving → Blank screen

• In Terminal:

    gsettings set org.gnome.desktop.session idle-delay 1800
  

to set the "Blank screen" delay to 30 minutes or 1800 seconds (or set any value in seconds).

Note: After making the change, in Settings GUI editor it will show "Never" as there is no entry for 30 minutes.

2. Set timeout of "Lock screen after blank"

• In GUI: Settings → Privacy → Screen Lock → Automatic Screen Lock Delay

• In Terminal:

    gsettings set org.gnome.desktop.screensaver lock-delay 600
  

to set the "Lock screen after blank for" delay to 10 minutes or 600 seconds (or set any value in seconds). Set 0 to lock immediately after blank screen.

To sum up

Combining these 2 parameters user is able to achieve for example:

• Blank screen after 10 minutes but lock after another 10 minutes (20 minutes overall to lock).
• Blank screen after 5 minutes and lock immediately.
• Blank screen after 30 minutes and lock after another 1 hour
• Any other combination depending on user's need.