The default timeout before the screen goes blank only gives you the limited options of 1, 2, 3, 4, 5, 8, 10, 12 and 15 minutes, then 'Never'.
If you want a screen blank timeout longer than 15 minutes, in Terminal:
gsettings set org.gnome.desktop.session idle-delay 1800
That sets it to 30 minutes (1800 seconds). That will be reflected in Settings > Power > Blank Screen after you change it.