Plan on twice a year for most Oahu homes, once before the wet season and once after it. Houses under heavy tree canopy or right against the mountains often need three, because debris arrives constantly rather than in one seasonal drop. Coastal homes can sometimes stretch to two, but they need the fasteners and seams checked each visit instead.

October is the single most useful month, because the wet season runs roughly November through March and you want the system clear before it starts. The second-best time is right after the rains end, when you find out what the winter actually did to the hangers and joints. Cleaning in the middle of the wet season works, but you are usually reacting to a problem by then.

It depends on the house. A single-story home with easy ladder footing and no canopy overhead is a reasonable job to do yourself. Two-story runs, tile roofs, steep pitches and anything over a lanai or a slope are where people get hurt, and where a missed corroded hanger turns into fascia repair that costs far more than the cleaning did.

October, if you are only doing it once. It puts the system in good shape immediately before the heaviest rain months, which is when a blockage does actual damage rather than just looking untidy. If you are doing it twice, pair October with April.

Watch the downspout, not the gutter. A trough can look reasonably clear from the ground while the elbow at the bottom is completely packed, and that is what sends water back over the front edge in heavy rain. Run a hose into the top of the downspout and time how long it takes to come out the bottom. If it backs up at all, the blockage is below, not above.