A gutter has one job and fails quietly. Full of leaves it does not overflow dramatically, it seeps over the back edge against the wall, soaking the brickwork where nobody is looking. The first sign indoors is often a patch on an upstairs ceiling or a tide mark on a chimney breast, by which point the wall has been wet for months.
When to do it
Late autumn once the leaves are down, and again in spring to clear what blew in over winter along with the moss washed off the roof. Add a look after any serious storm, because a single dislodged clump of moss will block a downpipe outlet completely. Overhanging trees mean three or four times a year rather than two.
Blockage or leak
Watch during heavy rain, which is the only reliable test. Water spilling over the edge along a length of gutter means a blockage or a sagging run holding standing water. Water running down the outside of a joint means the seal has gone. Water appearing at the top of a downpipe means the outlet is blocked rather than the gutter. The fixes are different and the diagnosis takes five minutes in the wet.
Doing it safely
Most gutter work goes wrong on the ladder rather than at the gutter. A ladder wants firm level ground, a person at the base, and the rule that you climb down and move it rather than leaning sideways to reach the next section. Never rest a ladder against the gutter itself, which is plastic and will flex or crack under the load.
Where the gutter is above a conservatory, a sloping roof, a basement drop or anything over roughly two storeys, stop. That is tower or scaffold work and it is not worth a fall.
While you are up there
Check the brackets. A gutter that holds water along part of its length has usually dropped at a bracket, and the standing water adds weight that drops it further. Refixing one bracket is a small job; replacing a sagging run that has pulled away from the fascia is not.
Look at the fascia board behind the gutter too. Soft or discoloured timber there means water has been getting behind the gutter for some time, and that is the more expensive problem beginning.
Downpipes and the ground
The job does not end at the gutter. A downpipe discharging onto a path that slopes towards the house is simply delivering roof water to the wall more efficiently. Check where it actually goes in heavy rain, and that the gully at the bottom is clear of silt and leaves rather than backing up and spilling.