breastfeeding all day

how long am I supposed to breastfeed on each breast? I feel like I'm breastfeeding all day.. every hour for 1-2 hours each session. My baby is four weeks old. I'm getting enough milk in but she just wants to stay breastfed all day. Its really hard to go out because shes constantly wanting to be breastfed. I check to see if shes hungry by testing her nursing reflexes. She falls asleep after 15-20 minutes so Ill walk away thinking shes full and then shell cry again to be fed. What am I doing wrong?