The better a dryer vent blows, the less lint builds up in it. Short vents blow better than long ones. Full size dryers blow better than smaller stack dryers or older dryers. Vents with a lot of turns and elbows blow worse and build up more lint. Very short vents attached to full size dryers may never need cleaning! Most vents, however, need cleaning every six months to one year, depending on the factors above.
A plugged vent will certainly cause a dryer to run hotter and increase the chance that something could catch fire. A dirty vent, however, is only one link in a chain of events that must occur for a fire to start. Other events may include: failure of thermostat and limit switches in the dryer, lint inside the dryer, a missing or damaged lint screen, and a crushed hose behind the dryer. Many fire departments and insurance companies do require that dryer vents be inspected and cleaned regularly to keep them from becoming a fire hazard.
Annual cleaning will reduce the amount of time it takes to dry a load. A dryer with a blocked vent can take 3 to 4 times as long to dry costing upwards of $3.00 per load compared to $.75. If your dryer is unable to let the hot air escape properly it will produce an unsafe amount of heat. This becomes a huge fire risk. When your dryer gets too hot a switch automatically shuts it down, Dryer Fires occur when this switch fails.
Most vents can be cleaned within an hour. Excessive lint buildup and the presence of a bird nests can take additional time.
Remove all items in and around your washer and dryer, the less stuff in the way the better. Also, remove all clothing from washer and dryer. If you have an aggressive pet please keep away from the technician.