There are several factors that contribute to lint accumulation (the topic for another post), and these factors vary significantly from house to house. However, no matter your dryer vent's configuration, the frequency for cleaning is based upon how much buildup occurs in your dryer vent. So how much accumulation should be found between cleanings? Not much more than a dusting. For straight sections of duct we look for no more than a dusting and for your transition duct, around your elbows (turns) and toward your termination cover there will be "balling." Since these latter areas are more restrictive they have increased accumulation and start to clump up (balling). Those balls of lint should be no more than a quarter in size (and hopefully not that large).
In order to target this level of buildup annual cleanings are preferred for most people. If your vent system is DIY-friendly (only exhausts a few inches through the side of your house and does not require an extension ladder to access), then by all means clean your vent system more regularly. However, for professional cleanings it usually turns out that an annual cleaning does pretty well in targeting the desired level of accumulation. If you go beyond this level you begin to flirt with the fire-hazard aspect of your vent system. In order to incentivize this proper maintenance we give an annual cleaning discount and offer, for a further discount, to schedule your next appointment a year in advance.