How long would it take to get to Greenland by boat?
That would depend on where in the world you were starting from, which you have not specified. After that, it would also depend on things like the type of boat and weather conditions.
What is an artifact of Greenland?
The Greenland Artifact is a so called unidentified object shown on google earth in the center of Greenland. It is, because of its size, just a colour run on the page, or possibly a reflection of the sun off the ice. To view it, go on google maps, and on satelite, zoom in on the middle of Greenland and you'll see it.
Denmark emerged as a unified kingdom in the 8th century.