All the Scandinavian countries are either in the EU or in the EEA so the rules linked by TweetTweet would apply, Ying&Yang. mgm is asking about a non-EU registered car.
I checked the booklet I have and basically the rules regarding importation are the same no matter where the car comes from, i.e. EU or non-EU. If the person is a non-Greek resident, or a Greek resident who works abroad for more than six months in a year, then the car can be brought in for a period of up to six months. After that it must leave the country or be immobilised off road. In the latter case it must be immobilised for 24 months. For somebody who is Greek resident it can be brought in for 1 month after which it must be registered in Greece.
The charges for registering in Greece are the same although there may be additional bureaucracy and costs to prove that the vehicle meets EU construction standards if it originated outside the EU. While the basic standards used to approve vehicles are international they are not necessarily applied in the same way in different countries. A friend who emigrated to the USA took his Harley Electroglide with him thinking it would be straightforward because it is produced in America. In reality it cost him almost what the bike was worth to convert it from EU specification to US specification.