timeCounting.groovy

import java.util.concurrent.TimeUnit;

def c = 0;
def w = TimeUnit.DAYS.toMillis(3)
def threshold = new Date().time - w

Jenkins.instance.getAllItems(Job.class).each { j ->
  for (b=j.lastBuild; b!=null; b=b.previousBuild) {
    if (b.timeInMillis<threshold)	break;
    c += b.duration;
//    println "${j.displayName} ${b.displayName}";
  }
}

println c/w