Gutters are troughs at the bottom-edge of roofs that catch the water before it falls by your basement.
Gutters need to catch water that would otherwise damage your foundation or erode the soil around your foundation.
Gutters should be well attached to the roof edge. They should be positioned so that they catch water in a drizzle or downpour. Gutters are usually fastened to a fascia board with nails (most common and work loose), long screws (never pull loose), and "hidden hangers" (shorter screws with clips that cannot be seen from below).
Gutters should be sloped in a way that water can flow towards "downspout outlets", which is connected to the top of the downspout. A 1/2 inch drop in elevation per 10 feet of gutter is a decent slope. Too fast of a slope (two inches in 10 feet) can cause water to overflow a downspout outlet. Gutters sloped away from the downspout can cause stagnate water to sit and eat away at the gutter, causing gutter drips, leaks and provide a breeding ground for mosquitoes.
Gutters are made from aluminum, steel (galvanized), painted steel, copper (fancy, expensive), plastic (cheap, often breaks and is flimsy) or many years ago, from old-growth wood.
Frankly, the correct answer is: as often as needed to prevent gutters from overflowing and downspouts from pluggin or clogging. It depends on how much rain you are getting, the slope of the gutter, leaves, needles, trees, toys, the size of the downspout outlet, the sharpness of bends and if you have gutter covers installed. In short, documenting when you clean them, how dirty they were and how often you cleaned them will give you an idea of future gutter cleaning requirements. If you don't clean the gutters, don't be afraid to ask someone you hired "How full were the gutters?" and use that information to write on your calendar when they should be cleaned next.
If you clean your gutters once a year, people often clean them after all the leaves have fallen.
If you clean your gutters twice a year, then they are cleaned during when the leaves are falling and after all the leaves have fallen. Or if you have trees with needles only, then you clean them twice a year, spaced 6 months apart, but not during summer (much less rain). Like April (before May showers) and October. Water will flow through needles until the needles rot and compact enough to restrict water flow.
Because you don't seen inside your gutters each day, week or month, write the task on a calendar to remind you. Tell your spouse. Emphasize its importance to avoid foundation damage to the house or basement flooding. Make a note of it. Make a note of when trees around your gutters start dropping their leaves. Watch the weather. If the leaves drop and a big rain is coming, get out there and clean the guttes. Either get ready for the job 1 weeks ahead of time or call a gutter cleaner 3 weeks ahead to set an appointment.
Downspout outlets connect the gutters to downspouts. Downspouts carry the water towards a drain. Drains carry the water (typically underground) into a sewer system or to a "dry well".
Downspout outlets can be square, rectangular or round. The larger the downspout outlet, the less likely it is to be clogged. Also, the larger the outlet, the more likely debris can get into the downspout to clog it or the drain. See also, leaf-traps.
See also DIY gutter cleaning
See also Basement flooding emergency
See also Preventing gutters from overflowing