Blocked gutters are among the cheapest problems to fix and the more expensive ones to ignore.
What actually goes wrong
Overflow runs back under the fascia and into the eaves. By the time it appears inside as a ceiling stain, water has been getting in for a while and the repair is a carpenter rather than a cleaner. Standing debris also holds moisture against the gutter itself, which is what rusts steel guttering from the inside.
The part most people skip
We clear the gutters and the valleys, flush the downpipes, then run water through and watch it drain before leaving. A gutter that looks spotless from the ground can still have a downpipe blocked solid at the bend, and you will not find out until the next real downpour.
Under tree cover around Warners Bay, Eleebana or the Newcastle suburbs with heavy gum canopy, twice a year is realistic. Open sites can usually go annually.