For me if anything at all is on the chips then it is about time. Some are not a fan of oiling but it does help to keep your chippies clean, if you play them dry after a wash, especially white clay, you will find it doesn't take very long at all for dirt to start building up again. Hence I avoid white if I can, I much prefer blue $1s if possible in a set as typically the White is a $1 workhorse chip.The short answer would be: when they're dirty.
When this point is actually reached depends solely on how often your set is used, for what amount of time and how dirty the hands of your players are/what you're serving during play.
As soon as black gunk (frequently called 'hooker juice') builds up in the crevices, I'd say it's time to clean.
Same - I try to really clean them (if needed) once when I initially get them. If they are a little dirty after a game, I might do a quick wipe down - but only if needed.Once. Then it's just oiling when they need it. Very rarely will any set get played to dirty again around here. It's takes a casino to really make the hooker juice flow.