yeah, my little boy lost 7oz in the first 5 days & took until he was 22 days old to regain his birth weight. (about 10 days is considered the average norm).
Feeding at first does vary HUGELY; everything is so new & it takes them a while to understand what's going on, what they need/want, how long it's been since they last fed, how windy they are, how sleepy they are, how they're latched on, what position they're in while feeding & lots more.
As the time goes on, they become more effective at feeding & more adjusted to what they want... You may find your LO will more in the evenings (cluster feeding) & will obviously want a lot more when going through growth spurts.
Harry (my son) fed anywhere from 5 minutes to an hour in the early weeks.
So long as you carry on feeding on demand & your baby comes off looking content & brings wind up well etc, chances are they're fine. So long as your LO is having plenty of wet & dirty nappies each day, hasn't got a sunken fontanelle, his skin turns pink again quickly if you pinch it, etc, he's probably getting enough at each of his feeds.
If you're at all worried, speak to your midwife/HV/GP for advice.