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.
Today, as I serviced one customer's house, the homeowner and I were discussing the importance of dryer vent cleaning. In our conversation I think she summarized it well: the main impetus for dryer vent cleaning is to mitigate the fire hazard, and shorter drying times and less energy usage are merely extra benefits.
You want to be cleaning the vent before excessive accumulation occurs, otherwise you begin flirting with the fire hazard. This means that if you follow the industry recommendations for intervals between cleaning (typically 1 year) you probably won't experience a dramatic change in drying times or energy usage (unless birds get in your vent!), but you will be following 'best practice' in order to mitigate the fire hazard.
It is always satisfying when I leave a home knowing that the owner(s) will experience substantially shorter drying times. It is impressive removing large amounts of lint or multiple feet of a bird nest because I know that there will be a drastic difference in airflow before and after cleanings. However, truth be told, I am much more satisfied leaving a home that has been maintained with regular cleanings and has removed marginal accumulation because I know that the family, house, and appliance are being safeguarded.
There is a reason that cleanings are recommended annually, and not every 5 or 10 years.