The short answer
Late November or early December, once the leaves are down — not October, or the gutter will be full again by January. Once a year is enough unless there are conifers or a plane tree nearby, in which case twice. If you can see a damp patch on the wall, the moment for prevention has already passed and you are paying for a repair rather than a visit.
A gutter does not fill evenly. Leaves, moss washed off the roof tiles and grit form a plug, and that plug always comes to rest at the same place: the outlet where the water enters the downpipe. That is precisely the one point you cannot see from the ground, and it is why people think their gutter is clear while water has been standing in it for months.
Water standing in a gutter looks for the shortest way out. With a timber fascia that is into the wood, and rot in a fascia board is a carpenter’s job at hundreds of euros a metre. With a masonry wall it spills over the edge and soaks in — first a dark band, then green growth, and in a hard winter frost damage in the pointing. Either way the repair is a multiple of the visit you could have booked.
That is why the running-water test is the most important part of the job, not the clearing itself. A gutter that looks empty from above tells you nothing: the plug is in the pipe below it. Anyone who empties the gutter without flushing it through until they can see it discharging freely at the bottom has left half the problem in place. So ask about that, whoever you have do it.
One thing you can judge yourself without a ladder: go outside during heavy rain and look. If water is running over the edge of the gutter, or less is coming out of the bottom of the downpipe than you would expect, it is full. That is a better indicator than a glance upwards on a dry day.
And the honest advice: if you have a bungalow with a low gutter and no trees nearby, you can do this perfectly well yourself with a ladder that is properly footed and somebody holding it. From two storeys up, and certainly with a dormer or a steep pitch, it is no longer worth it — most falls in and around the home happen off a ladder.
What to remember
- Late November, after leaf fall — October is too early.
- The blockage is almost always in the downpipe, not the gutter itself.
- A damp patch on the wall means you are already past the prevention stage.