Excellent question! The issue, as you're probably well aware, is the lack of reliable multi-centre trials that look at the efficacy of herbs like Andrographis as a preventive especially in the scenario of a new epidemic/pandemic. While it's immune boosting potential is fairly well-established [particularly TNF-alpha mediated] as far as I am aware there is no study that determines a recommended cycling pattern to use for such a purpose. Prolonged use does cause elevated liver enzymes and hence it does make sense to avoid that. The half-life of Andrographolide is about 6 hours so maintaining therapeutic plasma levels would require repeat dosing. What that means is that 'off' periods would imply no protection.So in the covid-19 scenario, in the absence of any reliable information a best guess would be to use herbs like Andrographis at times when the risk of exposure is maximal for example flights, conferences etc. With regard to cycling I am aware there was a study done a while ago using Echinacea for travellers at risk of respiratory illness on long haul flights. The cycling used there was 2 weeks prior to, during and 2 weeks after the flight. I don't know what the basis of that dosing frequency was. Some commercial preparations of Echinacea like the Triple Guard formulation recommend intervals of two weeks before repeating a course of treatment. So there might be something to that figure but I'm not sure of it's pharmacokinetic basis.So here we are...not much to go by really... In fact i was just yesterday recommending to my son who is going to be travelling a fair bit over the next couple of months that he take Echinacea, Vit C and Zinc once a fortnight and whenever he is on a flight...and I have no real scientific basis for my advice besides that one study from Griffith University which was for the common cold and URI's and certainly not for the covid-19 scenario! So I can only suggest that anybody considering such an approach should educate themselves on the potential adverse affects and drug interactions of any herbs they take and seek professional advise for their individual concerns.