Whenever I have something extra such as an apple/pear core, watermelon rind, etc. and my other creatures are fed I'll throw it in there, but that isn't too often. In reality you can keep them on straight rotting wood and leaves permanently, although the supplemental food definitely does help to keep your specimens healthy and growing.
So to answer your question, there isn't really a set agreement on how often you should feed your specimens. It depends on how many millipedes you have to feed, the size of the millipedes in question, whether other supplemental foods are being offered (i.e liquid/powdered supplemental foods, protein, and so on), etc.
I agree with @Arthroverts and @chanda, and would add that I aim for approximately twice a week for my millipedes, but it varies quite a bit, and I think that is probably much like it works out for millipedes in the wild.
thanks everyone i was providing some veg 24/7 but i think ill reduce it to 2-4 nights a week. i give them grinded cuttle bone its mixed in the substrate sprinkled on food and when i take food out left on substrate slightly. what sources of protein do they like?
my substrate is a mix of 3 different millipede subs its quite woody/composty so i mix in spagnum moss on top and hundreds of oak leaves/rotting wood throughout