depending on how your hair is (straight / curly) could depend on the time too. my roots dreaded up pretty quickly with no work, but i have pretty curly hair.
so... your saying that braids won't turn into dreads? and that thread won't eventually break down? well... whatever... I have breads that are turning into dreads and parts of my dreads that were braided and are now turning to dreads. and a friend had sewn part of her to help form the dreads and eventualy the thread will disinigrate from washing your hair and sweat, ect. and someone else I know had they're heir wrapped in yarn for long enough and they too turned into dreads.