My thoughts on this:
It’s not just about the amount of time you have been together - other factors come in to it too.
Do you live together and for how long have you lived together? If not, or if you have not lived together long, I’m not sure how you could know that a marriage to this person would be what you’d want and would work out in the long term. You wouldn’t know each other that well. You learn a lot about someone when you live with them.
Have either of you been through any major life events since you have been a couple? House moves, deaths of family members or friends, job losses or promotions, illnesses etc and how did you handle these issues? How did you handle the covid lockdowns? A person may want to understand better how all of these things are handled by their partner before committing to a marriage.
If you live together, do you rent or own a house? Could it be that they would rather spend money on a house than a wedding? I personally feel buying a house together is a bigger commitment in a way due to the amount of money involved, but that’s just my opinion.
What do you both want from life and have you discussed this? To travel, to have children, to focus on a career etc? These again could be factors that influence when/if somebody is ready for marriage.
How are your relationships with your own and each other’s families and friends?
How do you both handle finances and general life admin? In a similar way or very differently?
I’m not saying that this person is consciously thinking about all of these things. They may be or may not. But they are all things that may be influencing if they want to or are ready to get married, even if they don’t realise it.
It could be, for example, that one day they see how you react to any given situation within the family, and that makes them realise that they are ready for marriage. It also could be less cut and dry than that, and that if they do want marriage at some point in their life, it might be that they are the sort of person that really thinks things through before going ahead with them and just aren’t sure of a few things at this time.
Everybody is different. Some people are very in tune with their feelings and know what they want and when they are ready for things. Some people are over thinkers. Some people don’t want to get married, some do. Some people don’t feel the need to rush things.
I just don’t feel like it’s the sort of thing that can be given a timeline when there’s so many factors that can affect a relationship.