Gnome 3 doesn’t appear to offer any GUI control over the screen lock timeout.

So, to get current values:-

[snetting@lapper ~]$ gsettings get org.gnome.desktop.session idle-delay
uint32 300
[snetting@lapper ~]$ gsettings get org.gnome.desktop.screensaver lock-delay
uint32 0


And to set:-

[snetting@lapper ~]$ gsettings set org.gnome.desktop.session idle-delay 600


idle-delay is the time taken to blank the screen.
lock-delay is an additional delay before locking.