Eugene, OR
There's no fixed interval -- most homeowners in a wet climate like Oregon's find that cleaning once a year keeps moss and algae from building up on concrete, though shaded driveways or heavily trafficked walkways sometimes need it more often.
The same damp, shaded conditions that grow moss on a roof will grow it on a driveway or walkway, especially in sections that don't get much direct sun.
Mossy walks become a safety issue well before they're visibly unpleasant, which is why a lot of homeowners clean walkways on a different schedule than driveways.
