Hi R.,
Boy, I believe it really depends on the child and I think it veries for every baby. I have only 1 child, and she did a bit of all types of feedings. Long and slow, short and fast, or clusterfeeding. It was horrible to try and figure out. (bad foe scheduling things) I eventually found that since my boobs go everywhere my daughter goes, it didn't really matter how often, or how long, unless I was in the car. (I occasionally even had to pull over somewhere and feed her.) I had to grow comfortable whipping my boobs out where ever I went.
Don't know if this helps at all. I found that as long as she wasn't sleeping, she must've been eating. Good luck.
Oh, good tip. Bathrooms are kind of gross to feed in, but ok in a pinch. Dressing rooms are GREAT for feedings...quiet, they're almost everywhere, only women are in there, and you can stay in there as long as you want. (this might be more info than you want, but trying to figure out those feedings is hard to dial in...so try not to worry too much)