Congrats on the upcoming baby. I also failed first time BF and took a class my second go around. An exclusively breast fed baby is really not likely to go more than three hour. But many times people succumb, or need to supplement with a bit of formula. Formula fed and supplemented newborns can go 4 and 5 hours without eating. My first did and I thought it was great at the time. However, there are only 24 hours in a day, so I only averaged 6-7 feedings a day. Looking back its one of the reasons we failed at bf. You really need to get those 8 feedings in to stimulate your milk production. Do the math, that means baby needs to eat on average every three hours. For your milk supply, aim for 8 feedings. Now if baby sleeps 4 hours in a row in the night, and say he/she does this twice in the wee hours of the night so you only have to get up once instead of twice, keep that, its great, but do a little make up feeding in the day and space those feeding 2 hours apart. Keep those 8 feedings until your milk is well established.
In the same way that you should not let a baby go over three hours (give or take), you want to encourage them to make it 2 hours or you will be too exhausted and soar to continue BF. My second was a snack feeder, she couldn't make if more than an hour and sometimes just never really stopped nursing all day and all night. It turns out she had an ineffective suck, she ate all day and was still starving. I'm sooooo grateful my lactation consultant talk me out of demand feeding. I would never have make it. She told me that I needed to supplement just a touch to get that baby to go two hours or I was going to burn out. I'm so glad she saw how close I was to burn out because I was hell bent on demand feeding exclusively with BM and no formula. I was carrying my left over failures from the first child into my second experience and we were dealing with two different babies with different struggles. I'm so glad I put my trust in my lactation consultant. We only supplemented with a syringe full of formula, and as I was able we supplemented with pumped BM. All this bought me a little time for rest between feedings. Without rest your body will not produce milk. So yes, that 2-3 hours is golden if you want to BF. Failure is easily achieved if for some reason the baby cues outside of this tired and proven time between feedings. And yes sometimes you will need to demand feed/ cluster feed in the beginning until everyone gets the hang of it. The good news is that three hours between feedings is what most babies fall into without much manipulation. Should your baby not fall into that schedule easily, I would manipulate it by waking the baby (especially in the day time) or with minimal syringe supplementing (ideally with pumped BM) to get baby to go at least two hours between feedings (for your benefit). Just make sure you are working with a good lactation consultant. For us woman who struggle with BF, trouble shooting problems on our own is a bad idea. Take if from someone who struggled and failed, as well as struggled and succeeded to BF. I've been on both ends of the spectrum- a baby who goes 4-5 hours and a baby who couldn't even make it 2 hours all day and all night- both are recipes for failure. Working towards a 2-3 hour feedings does not mean "starving your baby for three hours".