Clean your dryer vent at least once a year, and more often under specific conditions. The U.S. Fire Administration and the National Fire Protection Association both identify failure to clean the dryer as the leading factor in an estimated 15,970 residential dryer fires per year. An annual cleaning is the baseline maintenance that addresses this.
Twice-a-year cleaning is warranted when: the household runs six or more loads per week, the dryer is more than 25 feet from the exterior termination, the vent makes multiple 90-degree turns, the vent runs through an attic or a crawl space, the vent includes sections of vinyl or foil flex hose (it shouldn't — only rigid metal or semi-rigid aluminum is safe), or the dryer has started taking longer than one full cycle to dry a normal load.
The lint screen alone is not enough. Lint bypasses the screen at the edges and accumulates on the drum seal, the blower wheel, inside the lint-housing duct, through the entire vent run, and at the exterior termination cap. A proper cleaning reaches all of those.
Signs the vent needs immediate attention: the dryer running hot to the touch, clothes coming out hotter than usual at the end of a cycle, visible lint at the outdoor vent cap, a burning smell during operation, the dryer tripping on its high-limit thermostat, or the repeated blowing of thermal fuses. Stop using the dryer and clear the vent before continuing.