If you exclusively breastfeed you'll milk will come in 3-4 days after birth. The more you feed the bigger babys belly gets and the more your breasts will know to produce.
I started at 12 weeks now im 30 weeks and its alot more.
I started producing colostrum at 17 weeks it just depends on the women its different for all :)
Your actual milk doesn't come in until after baby is born (usually within the first week after birth). But you might start producimg colostrum any day now, it's different for all women but the average time to start producing colostrum is between 20-30 weeks. I started producing colostrum at about 23 weeks, and some women don't even produce anything until baby is born but that is rare.