<p>5-8 years is the norm. </p>
<p>Totally depends on your field. </p>
<p>If you are aiming high, click on the following link, scroll down half way and choose your sub-field.</p>
<p>[Social</a> Sciences & Humanities - Best Graduate Schools - Education - US News and World Report](<a href=“http://grad-schools.usnews.rankingsandreviews.com/grad/socsci.html]Social”>http://grad-schools.usnews.rankingsandreviews.com/grad/socsci.html)</p>