Judges 2:6–15 provides some more details. There we are told that after the death of the elders who outlived Joshua, the generation which followed them did evil in the eyes of the Lord. If the lifetime of a generation is approximately forty years, then we can assume that Israel served the Lord for at least such a length of time. Of further interest in this verse is the fact that this is the only time in the Old Testament where we find the phrase the people served the Lord.
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31 Israel served the LORD all the days of Joshua, and all the days of the elders who outlived Joshua and had known all the work that the LORD did for Israel.