You have to adjust two things for this.
1. Configure the time needed to go for the display in Blank Mode
Open Settings Manager
Go to Hardware Section
Click on Power Manager
Click on Display Tab
There is an option Handle display power management. Enable that option by checking it. And change the timing parameter by using the slider with label Blank After, setting value whatever you want.
Remember, Less than 1 minute means Never.
Note: If you can't find Power Manager settings under Hardware section, you're missing xfce4-power-manager
package. Install it with this command
sudo apt install xfce4-power-manager
Here is the screenshot of the Settings Window
2. Configure the time must pass to active lock
This is controlled by Xscreensaver in Xubuntu.
Open Settings Manager
Go to Personal Section
Click Screensaver
While in Display Modes tab, at the bottom of it, there is a settings with label Lock Screen After [N] minutes. This controls the time required for the Lock to active after screen goes blank.
So, the calculation is, The time needed for the display to go blank + the time needed for the lock to be activated. (I set this 0)
Adjust this settings. Then wait for the display to go blank. Wait the time to active lock. There will be lock now.
Here is a picture of the settings window of it.
Hope this answers the question.