I think maybe she's saying ,'mum I'm done! Don't try and put me back on'. 20-30 minutes is a perfectly decent length. If you'd said 10-15 minutes it still would have been acceptable. Babies get faster as they get older - some quicker than others. I would agree that you should study her swallowing pattern. When it starts to slow down she's onto the hindmilk. It might speed up again if you have a new 'let down'. After a period of slower swallowing she should break off satisfied. I would try and follow her cues and see what happens. If she's coming off very quickly - bobbing off - then maybe it could be gas but it's an unusual time to start.