How Much Formula Should a 5-Week-Old Baby Normally Drink?

A 5-week-old baby is fed 130ml of formula each time, but often doesn’t finish it, sometimes consuming only 80-90ml, and occasionally eating a bit more before falling asleep or spitting out the bottle. Two hours later, the baby starts crying for hunger. Should one follow the book’s advice to feed every three to four hours, or feed the baby whenever they seem hungry?
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