The short answer
Twice a year, autumn and spring, with the ladder tied or footed and never leaned on the gutter itself. If it is above single storey, pay somebody.
- Do it in late autumn after leaf fall, and again in spring.
- Use a ladder stand-off so the ladder rests on the wall or the fascia, not on the gutter, which will break.
- Have somebody foot the ladder, and never over-reach. Move it instead.
- Scoop the debris into a bucket hung on the ladder, rather than dropping it.
- Flush with a hose afterwards and check the downpipe runs freely.
- Check while it is raining afterwards, which is the only way to see what actually overflows.
What will ruin it
Falls from ladders are among the commonest causes of serious injury in home maintenance. Anything above single storey, anything on uneven ground, and anything you are not confident about is worth paying somebody for.
Why it works
A blocked gutter overflows down the wall, and a wall that stays wet is the classic cause of penetrating damp inside. Gutters are the cheapest damp prevention in the house.
Household chemicals can be dangerous in combination. Never mix them, keep the room ventilated, and test anything on a hidden patch first. Full disclaimer.