Hi Missann, This is just the way I use it. There are surely going to be other opinions out there! Whatever works, is the bottom line
First, when I am repotting and I have trimmed the roots I do not soak the plant, I just pour the straight (3%) H2O2 on the roots. I pour a lot--maybe a good third of a 32 oz. bottle. Then I dust the cut ends of the roots with cinnamon. (I really don't know if you need that much H2O2, I am just operating on the " more is better" theory.)
I don't have a regular schedule for using the stuff. I use it as a soil drench whenever I bring a new plant into the house or whenever I suspect that something is wrong with a plant.
H2O2 and cinnamon are my first lines of defense because I try not to use chemicals unless there is no alternative. Brookn has a wonderful recipe on the forum for an all purpose cleaner and protector. Just mix 10 drops of Dawn dishwashing liquid, 8 ml. of cinnamon extract(a liquid in the spice aisle of your grocery store) and 2 cups water together.Voila!! You're good to go.
Hope this helps...