There is no set time scale. Every baby is different. Hearing one person say their baby has been sleeping though since 3 weeks old doesn't mean another baby will be the same. Some don't sleep through till they are a year or older.
I class 6 hours straight sleep as sleeping through. Doesn't matter if its from 10pm-4am or midnight-6am, if LO goes 6 hours, he's done great. Some nights he'll go 4 hours before waking, others 7, sometimes 8.
But one thing I have learnt and it seems to be a normal thing is that just when you think your baby has settled into a pattern, things change overnight. There is never not a reason, just its not always apparent to us why baby is waking. There are many reasons for LO waking at night. Your LO could be teething (even though you cannot see anything happening there is stuff going on), have wind that is unsettling him, feeling a bit hungry at night for some reason or just be too warm or a bit on the cold side. Might be that as he has woken a few times he has gotten into a bit of a habit with it and so just needs a bit of time to readjust.
You may find dropping the dream feed helps (if you've not already). Once past a certain age babies are able generally to go overnight without a late night feed (although some do continue with them). I didn't give our LO a dream feed when he began sleeping longer at night as it meant his body didn't learn to adjust to the longer sleeps without taking in food. As we grow we don't eat in our sleep. If Galen can go 6 hours without waking I didn't see the need for a dreamfeed to encourage this. In fact, when he feeds in the night it actually causes him to sleep worse the next night.
I think persevere with trying to settle without lifting from cot. Maybe use a mobile and music to soothe. Avoid eye contact, keep the room as dark as possible and avoid things like a nappy change unless you really *have* to. If LO wakes and won't settle maybe have your OH give water rather than a feed. I know a few people who said they did this and it did help. As my LO is breast fed we don't do this but we do make sure that OH sees to him if he wakes in the night. I don't go to him unless OH is really unable to settle him back down. Usually works and LO is off back to sleep within 20 minutes or so.