How often you clean your dryer vent depends on several factors.

What is the length of your dryer vent? 
Do you have an outside vent on your rooftop or on the outside wall of your home?
Are there bends or curves in the dryer vent; does it go through your attic?
How heavily do you use your dryer, once a week or several times per week?

Lint traps on your dryer collect approximately 75-80% of the lint from your clothes.  The balance is blown through your dryer vent and eventually your dryer vent may become clogged with this debris.  When this happens, your dryer works "harder" and you may even notice that the laundry rooms is very humid when the dryer is in operation.  Having a clogged dryer vent not only decreases the life of your dryer, but it is a safety hazard in your home.

When we service your dryer vent, the flex (as seen in the pictures above) is taken off and cleaned; in some cases may be replaced dependent on the condition of the flex.  Our technician goes up on your roof to check the outside vent, removes the screen and cleans this also.  A special brush is moved through the entire length of your vent to loosen the dirt, debris and sometimes even a nest.  Once this is completed, the vent is reconnected to your dryer, the dryer turned on and the loosened debris in the dryer vent is blown out through the exterior vent.  (We clean up all debris)  Our technician also takes the back panel off of the dryer and vacuums the inside of your dryer to remove lint that collects.  Notice the picture above with the electrical connections full of lint.  This is another safety hazard if not maintained regularly.