I don't have a rule as such but I generally don't use anything for more than 4 weeks at a time. The idea behind this is so the critters don't get used to what ever you are doing. If I'm using Neem, which I am at the moment, I take about 4 capsules x 260 mg capsules 2 times per day. If I'm taking Olive Leaf Extract, I was taking a good mouthful of liquid extract(tastes awful) about 4 times per day. If I was doing p'darco, I would make up a pot of tea and drink that throughout the day - boil dried herb (about a handful) in a pan of water and boil for 20 mins.