Hi! When I was nursing my first, I had sore cracked nipples, and the nurse at the hospital told me to put vit. E gel (you buy the little gelcaps at the pharmacy and break them open to get the gel out) and to put that on my nipples. It felt so good and they heales really fast. So I guess you could try that since yours are cracked too even though you are not nursing. For the nursing moms reading this, make sure to rinse it off before you nurse because babies don't like the taste of it.
About your question, I am sorry I can't really help you. I have been pregnant twice before but I don't remember exactly when I started to have colostrum. I think I was somewhere around 15-16 weeks.
I think I started leaking at around 15-16 weeks in my first pregnancy.
All women are different though and one woman may have certain symptoms at a certain part of their pregnancy, where another woman might not have that symptom until later.
I would ask your midwife's advice.
all the best.