It's vital to know that you're both going in the same direction in the relationship, and you both have to talk about it.
I'd known DH about a year before we became a couple and another year again before I moved in. It was somewhere towards a year after that, that we were chatting in the car and ended up talking about what we wanted for the future. The outcome of that conversation was that long term, we wanted to marry (probably eachother 
) but as I was in my early 20s, not yet professionally qualified and younger than him, I wanted time to feel established in the adult world first. I wouldn't say we were engaged then as it was too open, but were were clear that we were happily going in the same direction.
The actual engagement was a few years later. I'd been working for a couple of years and was feeling down about being looked over for an opportunity at work, and felt that we were sufficiently stable as a couple to seriously look at engagement/ marriage, and that there was no benefit to delay it because of work. So we browsed at rings, and a surprise bended knee moment followed within a couple of months and for various reasons a date set for two years later soon afterwards.
None of the conversations were premeditated or big things, they were a normal part of us communicating as a couple.
I have to admit that I fail to understand where couples live together, often for years without talking about what they want. It's so important to know where they agree, or have differences and need to review. And for reasons of biology and the impact that starting families tends to have on female career progression, it tends to be the woman that suffers by clinging on to false optomism that he may marry/ want children while her ability to have children wastes away with time. I'm not saying to rush, just that if you want to wait until 30+, 35+ be aware that you'll be doing it with the right person when the time is right, not then having to rush and waste another couple of years finding a suitable relationship against the biological clock because you were strung along for a wasted decade by an indifferent commitmentphobe.
Most of the weddings of dramatic relationships that I went to were followed by divorce a few years later. Sometimes with children involved (no clean break) sometimes before children happened. All relationships change, there's easy phases, flat phases, tension, but staying in because you've put in a lot of effort is a bad reason. The effort on both sides needs mutual reward and compromise. It's like the difference between maintaining a classic car and a clapped out runabout. One will give rewards for the investment of money, effort and experience, the other is a drain and best sent to the scrapyard.