Thats kind of a screensaver, and its actually good to stop your monitor burning out sooner rather than later. Although stepping the time up should be a good suggestion.
If memory serves me right, you use the command
set BLANK=numInMinutes - 15 is probably a good number. To turn make it so it doesnt do it at all, set it to 0 (zero). You can type that in the console, or to make it work everytime you boot (it doesnt just do it in fluxbox, it does it in the console too) put it in your ~/.profile (or to have it work for all users, in /etc/profile) - just whack it towards the end.
TTFN,
Scrapz
