Breastfeeding question

Hello ladies, I just had my baby girl today and am trying to breastfeed. She is latching and I’m told to let her nurse 15-20 mins on one nipple and then the next feeding switch to a diff nipple. Is this the best practice? Also, it has been over 20 mins and every time I try to slowly pull out my nipple , she’s sucking faster because she doesn’t want to let go. Should I continue to let her suck or pull out anyway ? How do I know she is getting something ?