The land - means here, as in Judges 1:2, not the whole land of Canaan, but the part concerned, probably the land of the tribe of Judah. Forty years, here and elsewhere, is (like fourscore years, Judges 3:30) a round number, perhaps equivalent to a generation.
Related Commentaries of Judges 3:11
Judges 3:11
11 And the land had rest forty years. And Othniel the son of Kenaz died.