6 min should be enough.But there are other methodes to stretch the tunica in diferent ways.You should stretch the tunica while erect (
http://www.mattersofsize.com/forum/penis-enlargement-forum/87054-twisted-erect-stretches-the-tunica-transformer.html , you can't stretch the tunica the same way when your dick is flaccid ) and also have a mini stretch session after a girth workout once a week ( read
SRT ) , again ,you're stretching the tunica in that expanded state,unachievable when your dick is flaccid or erect.If you do all of these exercises ( not a lot,5 min of each ) you're going to destroy your tunica.