This is the age old question when it comes to gutter cleaning.
There is no real answer as to how often you should have your gutters cleaned other than at least making sure it’s done annually (once a year) to keep on top if it and stop problems before they occur.
Likewise most home insurance policy’s require you to have your gutters cleaned at least once a year otherwise they can refuse to pay out for any damp, structural or even cosmetic issues caused by leaking or overflowing gutters which can get very expensive.
Some properties will need to be done more frequently than on a year basis, we have a lot who are done every 6 months and even a few that are done every 3 months because of the amount of debris that builds up quickly in them. But each property is unique, the rule of thumb is to keep on top if it and don’t wait until after there is a problem to get them cleaned and sorted.