How often do you spray with your solution?
Overall, it amounts to when I feel like it or think I need to. Peroxide only nixes spores that are present at the time it is sprayed. Afterward, it is just water. So every thing could be spayed everyday all season long.
My 'best known' fungal issues have arisen in dabbling with rooting cuttings. I put them in standard plastic '1 gallon' nursery pots and cover it with a 1 gallon storage bag. I found that leaves tended to get consumed by some kind of fungus if I didn't pop the bags off and spray about every 3 days, so that has become my mantra for 'how often'.
I've been fortunate to have few fungal problems, so it has been relatively easy for me to catch on to when fungal issues tend to arise and what ones tend to show up in my collection/garden. For instance, I have crab apples, pears, and junipers. When ever I see leaf spots that might be rust, I remove the affected leaves (they are just spore sources for more infections) and spray all of these species in my collection every 3 days (or so) for at least 2 weeks and then quit when I no longer find any possible rust spots. I make a point of looking over each of my trees prior to possibly watering - just saying 'hi, how are you doing?' If it isn't bugs and seem suspicious I execute the standard routing (pluck leaves and spray), yadda, yadda, yadda.
I have a couple of plants that have a strong tendency for the leaves to brown when water is left to stand on them. A peroxide spray after watering (or after rain) seems to nix this trouble, so I spay anytime the foliage gets wet. Some azalea and camellia blossoms discolor in similar circumstances - I similarly spray those blossoms just so I can enjoy them longer (or so I think as I've not been doing it for enough years yet to be sure that it is a real and not imagined effect).