how long to feed?

Erin
My little one will be a month old on the 24. I'm having a hard time figuring out if she's eating or just hanging out. I have the worst time telling if she is swallowing or not. We usually spend between 10 and 20 minutes on the first breast, then maybe 10 minutes on the second. Sometimes it's longer. I've been told to only let her feed 10 minutes per side and to keep switching back and forth, but if I do that she loses interest. How long do your 1 month olds tend to feed? I know everyone is different, but I'd appreciate any insight!