No real rule of thumb. If you're down on power, or your mpg changes dramatically, you may be a candidate for an intake cleaning.
In the A3/B4 cars, I usually see the worst clogging in the "body" of the intake, near the ports to the head. You can't see this by looking into the intake neck.
In the A4 cars, I usually see the worst clogging in the EGR/antishudder and intake neck. The body usually has some clogging, but not to the degree of the neck.
Y'all know to take the car to 4K RPM at least once a week (preferred daily) to cut down on stuff in the intake? I hold it for 30 seconds or so. AKA the "Italian tuneup". Very important to do to limit intake buildup. My wife relishes this moment every day, as it allows her to attack an Interstate onramp.
I also think a solution to manage oily vapors before they get into the intake tract is necessary. I use the ONCCV, but they are no longer available for the A3/B4 cars. You can make a homemade rig that will at least limit the amount of the vapors.