With a first baby, it can seem so hard to tell if your baby has finished feeding or not. I have been asked about this a lot. Certainly the amount of time isn't a reliable indicator, as some feeds will take a shorter or longer time than others - it is normal for the length of a feed to vary through a 24-hour period. Usually, your baby's hands will give you cues about whether she needs to continue the feed or is finished. While there can be exceptions, I have actually found these very rare.
At the beginning of a feed your baby's hands will be bunched up, often tightly into little fists. Anyway, they'll be closed, not open and floppy. As the feed progresses, the hands become more relaxed, often a gradual process. If your baby stops feeding, but stays at the breast, you wonder, 'Is she just taking a break, or is she done?' A few babies may release the breast and go to sleep, but other babies just stay on!
When in doubt, it can help to look at your baby's little hand to see if the fingers have opened up, or to lift her hand gently to see if it is floppy. To give an example, a mother I know found this a useful guide with her first baby. He usually came off the breast and grizzled after a feed, and checking if his hand was floppy helped her to tell if he'd finished the feed and was 'just being him', or it he needed to resume the feed. I often find this cue that a baby's hands give is reassuring to mothers.