Agree with pp, the length of feeds can vary greatly from baby to baby, mine were both sooo different. DD would take over 30minutes whilst DS was a 7minute per side feeder.
The important thing is that you let your LO finish the feed herself, if she's no longer interested, she's turning away, then that's it. You can burp her and try to offer the breast again But I wouldn't worry, especially seeing she's gaining weight well.
xxx