Jenkins Builds Duration over the last month

gistfile1.groovy

def summarize(t) {
 def minutes = 0;
 def now = System.currentTimeMillis();
 def month = 30L*24*60*60*1000;

 t.items.each { job ->
   if (job instanceof Job) {
     def b = job.lastBuild;
     while (b!=null && (now-b.timeInMillis<month)) {
       minutes += b.duration;
       b = b.previousBuild;
     }
   } else
   if (job instanceof ItemGroup) {
     minutes += summarize(job);
   }
 }

 return minutes;
}

println (summarize(Jenkins.instance) / (60*1000));