title: Length of a datetimeindex in python
body: <p>I just cant find the answer and I know pandas eats problems like this for desert.</p>
  
  <p>I have a <code>datetime index</code> and want to know its length, in years:</p>
  
  <pre><code>idx=pd.date_range('2011-07-03', '2015-07-10')
  </code></pre>
  
  <p>expected output:</p>
  
  <pre><code>4.0191 years   (4 years and 7 days)
  </code></pre>
  
  <p>If I do:
  <code>idx[0]-idx[-1]</code> I get an output in <code>days</code>, but id like it in years</p>
  
  <p>Sorry: couldn't locate on <code>panads</code> docs </p>