Albert Barnes Commentary Judges 13:1

"And the children of Israel again did that which was evil in the sight of Jehovah; and Jehovah delivered them into the hand of the Philistines forty years." — Judges 13:1 (ASV)

The Philistines have been mentioned as oppressors of Israel in (Judges 3:31); (Judges 10:7), (Judges 10:11); and the Israelite worship of the gods of the Philistines is spoken of in (Judges 10:6). But this is the first time that we have any detailed history in connection with the Philistines. They continued to be the prominent enemies of Israel until the time of David.

Forty years - The Philistine dominion began before the birth of Samson (Judges 13:5), and was in force during Samson’s twenty years’ judgeship (Judges 14:4); (Judges 15:20). The 40 years are, therefore, about coincident with Samson’s life.