Albert Barnes Commentary Judges 2:7

"And the people served Jehovah all the days of Joshua, and all the days of the elders that outlived Joshua, who had seen all the great work of Jehovah that he had wrought for Israel." — Judges 2:7 (ASV)

If Joshua was about 80 at the entrance into Canaan, 30 years would bring us to the close of his life. The "elders" would be all that were old enough to take part in the wars of Canaan (Judges 3:1–2); and therefore, reckoning from the age of 20 to 70, a period of about 50 years may be assigned from the entrance into Canaan to the death of the elders, or 20 years after the death of Joshua.

The great works of the Lord - The overthrow of the Canaanite nations.