Albert Barnes Commentary Judges 3:11

"And the land had rest forty years. And Othniel the son of Kenaz died." — Judges 3:11 (ASV)

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 eighty years, (Judges 3:30)) a round number, perhaps equivalent to a generation.