Well, my experience is that opinions differ greatly, morticia. I've been told that it's okay to take it forever (as it happens, I do disagree with that), I've been told to take it as and when I think I need it but the advice I agree with is the one that I follow.
Take it over the winter months to help with colds and bugs. I've been doing this for years now with good results.
Oh, and the best echinacea to buy I think is Boots own. I've always gone to the Health Rack before but hubby got a particularly bad cold a few weeks ago and I'd got none in so I bought the first I could get which was these. They've been marvellous.