mine has been there since about 8 weeks! but its very faint and hasnt darkened at all since it appeared I was wondering the same thing. I guess everyone is different!
Never had one and this is my 3rd, some women are also more prone to getting it than others, the absolute weirdest thing this pregnancy for me though is I have it on my nipples as weird as it sounds, I guess it happens...I have a very dark line that gos from my actual nipple to the out side of the aereola...I'm hoping with all the breast changes I've had this pregnancy vs my last two that it's a good sign that I will have better luck breast feeding.