breast feed every three hours?

can someone please clarify... they say breastfeed every 3 or so hours. if your baby takes 5mins to wake up and have his nappy changed, one hour to feed (i pull him off, burp him but he still wants more), change nappy and settle (up to an hour sometimes) that sometimes only leaves 45mins of sleeping before he is due a feed... are people waking thgere baby when they may have had only a small amount of sleep? my boy is a good weight and at 7 days was almost back at his 4kg birth weight but the midwive suggested 3hr day feeds, 4at night but so confused due to his loing feeds. shortest feed was 41mins, longest 1hr20.