How much should I express

My little one is great at taking from me or the bottle and I want to be able to start getting some flexibility and some exercise in. She's 4weeks old and weighs 8lb 10oz well over birth weight now. She nurses anywhere between 6 and 10 times a day.

I've expressed occasionally but am never really sure how much to express to make sure she's getting enough. I'm worried that days she only feeds 6 times isn't enough for her? Advice???

Does anyone have a guide for how much to express?