I suspect that with a Debian/Fluxbox combination, there is still an xorg.conf file.
In there, you can specify such things as DPMS, monitor suspend-time, off-time, and such things. How well these options will work, depends in part on how modern/unmodern your monitor & video card are, and probably more on which video driver you use for your video card. Not all hardware on the planet properly responds to these options, but basically, to answer your post, the xorg.conf file is I believe what you want.
Try `man xorg.conf` in a console.
