I think the 2 year rule works well in both directions.
If you've been dating
someone seriously for two years, and one of you honestly doesn't know if you see it as a relationship
that's on the long term track, then it probably isn't, and hanging around waiting for another four or five years isn't going to help.
On the flip side, 2 years is about the point where strong infatuation/lust tends to wear off. So if you fall strongly head over heels for someone, it can be very wise to not get married until you figure out if you actually like and can live with the person, rather than realizing after you've married and had a kid that the main thing holding you together was infatuation.
That doesn't mean that after 2 years you need to be engaged, though, or even that a relationship
has to lead to a commitment, just that by two years you're generally either on the same page, or heading to heartbreak.
A couple of pieces of advice. When you have a conversation about your future, be very careful to listen to both what he says and how he says it. "Yeah, we'll get married, someday. Maybe when we're 30? 35?" technically says he's interested, but the way it's said implies "Marriage? That's not even on my radar right now!"
Secondly, when you're in a holding pattern relationship
like this, be very careful not to get too entangled. If marriage is important to you, settling for 'marriage lite' is likely to leave you unhappy in the long run. So don't move in together to 'save rent', secretly seeing it as the next step on the path to marriage. Keep your finances separate. And definitely, definitely, make really sure you don't get pregnant. Getting married and having kids are not things that make a shaky or uncertain relationship
better - instead they'll simply make the flaws bigger.
Third, ultimatums are most useful when they're internal. Saying to yourself "If this relationship
hasn't moved anywhere in the next year, I'm going to move on with my life" can be very useful to you. Saying to your BF "If I'm not wearing a ring in 12 months I'm leaving" can have unintended consequences - first, if it doesn't happen and you really didn't plan to leave, and second, if you've got your ring, but it comes from someone who feels trapped. A friend of mine tried the "I need a ring or else" approach with her BF of several years, and he dumped her by text message not long afterwards.
Finally, live your life as if it's happening now. If you concentrate on what will happen in the future when you get engaged/married/have kids/move in together, it's easy to lose track of the fact that your life is happening already, not in the future some time.