The simple answer to that question is … well there is not one. Breastfeeding should not hurt. At least not more than a mild sunburn. And only at the beginning of the latch. And only for the first few days. If it does hurt more than that, ask for help and ask for it soon. The sooner the problem causing the pain is corrected, the better.
What might cause nipple pain? An incorrect latch and/or position. Engorgement. Infection due to nipple damage. Skin conditions.
The main reason is the first one listed, incorrect latching or positioning. This can usually be fixed with some simple adjustments. (Remember the basics: nipple to nose, tummy to mummy, wide open mouth, patience and persistence!) Read up on latching, watch a video, ask your support group for help or talk to a lactation consultant!
Breastfeeding shouldn’t hurt!