I use to do about 20 minutes... I don't boil it anymore, I just mix DME with water and pitch my vial. Contamination could result in this, however, I have never had a problem with it.
I've done 7-8 minute vigorous boils for years with great results. Not saying it's better than 10 or 20, but it has worked for me. I just don't think you need extended boiling times to effectively sanitize the typical 1-2 liter starter wort.