How long does hair have to be to put in clip in extensions (full head)?

usually its supposed to be around 2 inches, thats what i heard..try a href="http://www.hairextensiostore.biz" rel="nofollow"http://www.hairextensiostore.biz/a they have full head sets..plus they have a sale on 18" for i think its like $94 i just got it for my sister and she loves it...