Re: need advice...
Sadly, I don't think that any states have requirements about reduced work days for breastfeeding mothers. There certainly isn't any kind of federal regulation.
If you can advocate for your 15min break (do smokers in your workplace take a break(s)?), then do so. It's your right. What you do with your 15 minutes is your decision, as long as it doesn't break any laws (i.e., illegal drugs).
One day of missing feedings shouldn't lower your milk supply. And, since you've said that you're schedule varies, you shouldn't be falling into a pattern of missing the same feedings each day. If you do think that your supply is lower, you'll help it bounce back by nursing more on your days off.
Ideally, babies should be drinking 1oz to 1.5oz of expressed milk for each hour they are separated from you. This varies, depending on baby's provider, solids given, and how much baby would prefer to wait to get it "straight from the source". So, your 6-8oz that you get at the end of a shift really is pretty normal.
You don't mention what kind of pump you're using - please make sure it's not one made by a company that also makes formula. So, stick with Ameda, Avent, or Medela products, mostly.
By going 8hrs without nursing, I'd be most worried about getting plugged ducts (which you mentioned). Nurse/pump as soon as you can, and treat any plugged ducts as soon as you think you might be getting one.
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