I've not done much reading on this, but most of my mommy friends say that by 4 months (or sooner for girls), most babies sleep at least one 6-hour stretch a night. Formula-fed babies sleep for longer periods sooner than breast-fed ones, I know, but still. 6 hours of sleep is an unimaginable luxury. I don't even know if I can sleep that long in one stretch anymore!
My baby never slept long stretches to begin with. A neighbor who gave birth to a girl six weeks before I had my son said her newborn daughter slept 4-6 hour stretches and had to be woken up for feedings. When she first started solids, she slept 10 straight hours. In comparison, my son went through a stage of nursing every hour in his first 6 weeks. Otherwise, his normal schedule would be nursing every 2 hours for the first 3-4 months, and now every 3-4 hours. At night, he sleeps one 3-4 hour stretch then wakes every 2 hours or more often, not always too feed, though. Lately there is always an hour between 3 and 5 am when he is awake, trying desperately to fall back asleep, and having a difficult time doing so. Poor little one.
Size-wise, he's at the top of the percentile charts in height, and in the upper 50% in weight, so he's certainly getting enough sleep. Attempting to feed him larger quantities at one time to make him sleep longer just results in him throwing up from being overfed, so that doesn't work. The little reading I have done says that babies who sleep in their parents' beds take longer to sleep through the night as well, and as my son sleeps between my husband and myself, this could be a factor. But having him next to me makes nursing in the night so much more convenient, plus our tiny bedroom doesn't leave much room for a cot.
Maybe I'm just making excuses, but I'm sure the night will come when he sleeps as long as the average baby. At least I hope it does.
Meanwhile, I, who used to sleep soundly for 9-10 hours at a time, have long since gotten used to short stretches of sleep. A full night's rest, or even 5-6 hours, is now the stuff that dreams are made of.