Breastfeed Duration

Anneila
My baby is 3 weeks y.o. But every time I try to breastfeed her, she just drinks it for 5 minutes. The rest of it, she sleeps. I try to wake her up every 1.5 hour, but she still drinks for 5 minutes. Is it normal? Because I heard, baby must drink at least 15 minutes. I'm just afraid that my girl doesn't get proper amount of the milk.