Bottle Nipple Size

How do you know when you should change the bottle nipple size? Our little one (6 weeks old) is using the slowest flow nipple (size 0) currently and it takes her forever to eat (at least 30 minutes). Sometimes she gets fussy when eating like she is frustrated that it's not coming out fast enough but it's not with every feed. I've tried giving her the next size (size 1)recently but it was flowing out of her mouth. Does she just need to practice with it or is she still not ready for it?