It depends mostly on your
juice. Sweet dark juices generally tend to gunk up attys faster than clear juices. Some people say that high VG juices gunk up
coils faster than PG juices, though I don't completely subscribe to that. There may be a tendency, but I won't call it a rule of thumb.
I usually clean a 306 atty around every ~30ml of
juice consumption. YMMV.