That's why I also go get a "poke around and look" checkup every year. The X-ray is to spot deeper things, but you can't do that every year without the x-rays becoming a risk factor themselves.
In the United States, it is routine to get your mouth X-Ray’ed every 6 months when you go in for a cleaning. Dental plans include these checkups at no extra charge.
So it sounds like it's the individual dentists judgment, and their organizing body says that they base it on the occurrence of dental problems in the population the clinics they oversee, with some adjustments based on patient age and risk factors.
So according to Danish authorities, you should be safe getting x-ray'ed every 6 months. They just don't recommend it, since it's a waste.