After Deborah died, and Barak as well, Israel did what they often did during the days of the judges, when a judge died,

Then the children of Israel did evil in the sight of the LORD. So the LORD delivered them into the hand of Midian for seven years… (Judges 6.1).

At other times, the Lord delivered Israel to their enemies for times different than seven years. Why did He deliver Israel to Midian for seven years?

The Bible does not say, from what I can tell, and since this was not a pattern, that is, since He often delivered Israel for times other than seven years, I am inclined to conclude that this period was incidental.

It just might be that it was seven because that was how long before Israel sought the Lord again, and before Gideon was ready to take on the role of judge.