Real-time means that the software is continuously busy updating whatever info it generates, without (a need for) user interaction. Windows services are a good example of real-time software.
Software that uses user specified intervals is by definition not real-time.
There is software that uses a mix-and-match of these concepts (of course), but explaining that is out of the scope from the question.
Crap, I forgot to mention the software I use to find out where all the space on my hard disks goes. SpaceSniffer
gives a nice graphical overview and uses (some) windows explorer functionality to clean up disks. I like the graphical concept it uses. It doesn't support user intervals per se, but together with the Windows Task scheduler there should be options.
Try it and see for yourself, it's a 1.5MByte download and freeware.