It depends on how strong of an infestation you have so I would continue following the instructions until you see progress; 2-4 weeks maybe? The hot summer temperatures should help self correct the problem soon. Watering the soil and not the leaves will also help but I would not be surprised if the powdery mildew originated from the nursery or wholesaler. They place the pots close to each other so watering takes less time (HD).

I doubt you have much of an issue - powdery mildew is more of a cosmetic concern than it is a serious health issue with the plant.

I'd wonder how you watered the new plantings. Newly planted trees and shrubs do need regular deep waterings, especially if planted in summer or if you are experiencing high temperatures. But you want to water deeply and thoroughly at the root zone, not sprinkle the foliage. Watering daily may or may not be necessary - you will need to check the soil moisture physically by testing with your finger down in the root zone area. Allow soil to dry slightly (but not completely) between waterings. Powdery mildew is often a cultural problem associated with dry soils and high humidty - lightly watering the foliage but not soaking the soil will create an ideal situation for this fungus to develop.

You should know that fungicides are only preventatives, not curatives. IOW, any affected foliage now will not miraculously become clean and free of disease. All the fungicide will do is prevent unaffected foliage from becoming infected. And be sure to apply early in the day or in late afternoon/early evening so it does not react with the sun to create other problems.

FWIW, I would not recommend a rock or stone mulch for hydrangeas. They would much prefer an organic mulch - bark, compost, etc. - which would moderate soil heat and conserve soil moisture better. Removing the flowers is your choice - they have only been damaged by overhead watering reacting with sun and/or heat. Since PM is a foliar disease, flowers are typically unaffected.

Thank you both for your comments. I actually did water not only the soil but also did sprinkle the plant and flowers as well. OOPS. Now I know better.

If I cut off the flowers, should I expect to see new blooms this season, or is it pretty much done blooming for this year?