The general rule
Most homes benefit from a full vent cleaning once a year. Large families, long vent runs and pets usually need it more often.
Lint screen vs. vent
The lint screen catches most lint, not all. The rest builds up inside the duct where you can't see it.
Check the duct material
Plastic or foil accordion ducts sag and trap lint. Rigid or semi-rigid metal duct is the safer choice.
General information only, not legal or professional advice.