How long per BF session?

[writingislove]

For moms who are nursing their newborns, how long do they usually BF in one feeding? And do you notice some days they nurse longer than others? My 2-week-old usually eats for about 20 minutes max, but the past two days she has only done about 10-12 minutes max per feeding. She doesn't appear to be too hungry, so I'm hoping she's taking enough, but it still concerns me because 10-12 minutes isn't very long. Thoughts/experiences??