When running without a graphical desktop, Raspberry Pi OS will blank the screen after 10 minutes without user input, e.g. mouse movement or key presses.
The current setting, in seconds, can be displayed using:
cat /sys/module/kernel/parameters/consoleblank
To change the consoleblank setting, edit the kernel command line:
sudo nano /boot/cmdline.txt
The file /boot/cmdline.txt contains a single line of text. Add consoleblank=n to have the console blank after n seconds of inactivity. For example consoleblank=300 will cause the console to blank after 300 seconds, 5 minutes, of inactivity. Make sure that you add your consoleblank option to the single line of text already in the cmdline.txt file. To disable screen blanking, set consoleblank=0.
You can also use the raspi-config tool to disable screen blanking. Note that the screen blanking setting in raspi-config also controls screen blanking when the graphical desktop is running.
Raspberry Pi OS will blank the graphical desktop after 10 minutes without user input. You can disable this by changing the 'Screen Blanking' option in the Raspberry Pi Configuration tool, which is available on the Preferences menu. Note that the 'Screen Blanking' option also controls screen blanking when the graphical desktop is not running.
There is also a graphical screensaver available, which can be installed as follows:
sudo apt install xscreensaver
This may take a few minutes.
Once this has been installed, you can find the Screensaver application on the Preferences menu: it provides many options for setting up the screensaver, including disabling it completely.