Depends on the baby – a few start out as champion feeders, many take two or three weeks, some need as long as a month. They're all individuals.
I don't think there's much point it working at it for more than 15 minutes at a time in the first few weeks if the baby really can't stay awake. Just feed her when she does wake up. She is doing exactly what she needs to meet her newborn needs, and knows nothing of adults schedules.
As she stays awake longer, you can gradually try to extend the time between feedings if she hasn't started self-regulating by then (many babies will). But my baby girl couldn't keep down enough food to hold her for even an hour for her first month or so, or for much more than two hours between feedings for her first six months. I learned not to fight it, or it would have had me frantic.
In those days, pediatricians were saying not to feed more often than every 4 hours. Absurd – she would have been miserable, and wasted away. Every time I coaxed her to take a few more swallows, she always spit up a few extra swallows worth. Her little tummy knew how much it could hold.