Before answering this question let’s look at the most common problems that occur when gutters don’t get cleaned.
Loose gutters are the result of rotted leaves, debris and small twigs accumulating weight and pulling at the system, potentially damaging your trim or siding.
Ice dams are formed when gutters become clogged and water flows over the gutter and freezes.  This can cause catastrophic damage when the water begins to thaw and travel to the inside of your home.  Having ice dams steamed off is a much worse investment than just having a high quality heated gutter guard installed by a local professional.
Pests like rodents, ants and mosquitoes find debris ridden gutters to be welcoming habitat.  With the continued uptick in cases of mosquito transmitted viruses such as EEE and West Nile virus, homeowners would do well to utilize a micro-mesh gutter guard that completely prevents pests and all debris from entering gutters in the first place.
Given these issues, one has to consider their environment and tend to cleaning accordingly.  It is advisable to clean your gutters at least bi-annually.