Why twice a year
Once in late fall after the leaves and the first Santa Ana winds have dropped everything onto the roof, and once in spring after the winter storms have washed whatever was left into the gutters. Houses under pines, eucalyptus or palms may need a third pass. Clean gutters carry water; full gutters hold it.
Full gutters overflow onto the landscaping and the foundation, and their weight pulls them off the fascia.
What goes wrong when you skip it
Standing water rots the fascia behind the gutter. Overflow wets the stucco and stains it. Water backing up under the eave course of roofing wets the deck. Mosquitoes breed in the standing water. In colder places clogged gutters make ice dams that push water under the shingles; here the equivalent is the winter downpour that has nowhere to go.
The repair for a rotted fascia and eave costs far more than years of cleaning.
How to do it
A stable ladder on level ground, gloves, a bucket hooked to the ladder, and a plastic scoop. Work from the downspout end outward, scoop the debris, then flush with a hose to check the downspout runs free. If it does not, a plumber's snake from the bottom usually clears it. Check the gutter slope while the water is running; it should drain, not pool.
Never lean the ladder on the gutter and never overreach; move the ladder instead.
Check while you are up there
Loose gutter spikes or hangers, seams that drip, downspouts that have separated at the elbows, and the condition of the fascia paint. Look at the edge of the roof for lifted shingles or slipped eave tiles. Five minutes of looking saves a lot of guessing later.
Note anything you see and mention it at the next roof inspection.
When to hand it over
Two-story houses, tile roofs where you cannot safely reach from the ladder, anyone who is not steady on a ladder, and anyone who keeps meaning to do it and does not. Gutter clearing is part of my yearly protection plan for exactly those people.
The point is that it gets done, not who does it.
A note on downspout extensions
Where a downspout ends at the foundation, add an extension or a splash block that carries the water several feet out onto the lawn or into a drain. Most of the wet-garage and damp-slab problems I see start with a downspout dumping straight down at the corner of the house.