It depends a bit, mostly on stocking level, substrate type, if you have heavy poopers etc.
Generally speaking, in lightly stocked, not overfed, heavily planted tanks there is little need to clean the substrate.
In my 36 gal tank, I use a gravel vac about once a year to lightly go over areas where mulm accumulates. Keep in mind that the "dark gold" turns into plant nutrients over time, and as long as you do regular water changes, has no detrimental effect on water quality. Depending on your substrate and on the size of the particular particles you may not see anything on top of the substrate as it falls into the cracks. In my example, I use flourite as a substrate, and small fish like tetras produced small sized packets that disappear below the substrate surface.
The "do not disturb" suggestion is to be taken with a grain of salt too. Obviously don't suck out plants through the gravel vac, but carefully cleaning around plants, between ground cover, etc will not upset things. It's just not necessary that much (with reasonable stocking etc see above).
Of course, if you use root tabs or other means to fertilize your substrate you don't want to deep clean!