This behaviour is used as a screensaver for the terminal.
To disable it Edit /etc/default/grub and add consoleblank=0 in the GRUB_CMDLINE_LINUX_DEFAULT option:
/etc/default/grub
consoleblank=0
GRUB_CMDLINE_LINUX_DEFAULT
GRUB_CMDLINE_LINUX_DEFAULT="consoleblank=0"