I'm not a pill-popper of any kind, but someone told me that echinacea gives a temporary jump-start to your immune system, the best way to use it is in a double dose at the very first sign of a cold. Her advice was that continued use served little purpose.
I've used it that way a couple of times this Winter (two tablets at bedtime, two the next morning, then no more), and in both cases the symptoms went away.