Being a girl, when I read your title, I instantly remembered being concerned because my period started and I was going on a date. lol
Here's my old-school thoughts on dating-going-to-marriage. Back in the olden days, if you weren't married after 18 months, start worrying the relationship is going no where. Still dating after 7 years meant the guy has no intention of ever marrying you. Dump him. (Probably better to dump him if it's not going anywhere after three years. And, oh, btw, and engagement ring without a set date being worked out is not engagement. It's dangling the woman.)
Truth be known, when you know he is the one for you, and he knows you are the one for him, that's time for engagement. And not merely engagement -- it's time to set a date.
If you don't know if he is the one after 18 months, or he doesn't know you're the one, that is the answer. They're not the one. If you don't know someone you're dating after 18 months, either you can live without the person just fine, or he's hiding who he is, so you don't want to marry him.
Hubby told me on the first date that he was out to find himself a wife. I already liked him -- a lot -- but I wasn't planning on getting married. I was by the end of that date. He was the one. (And truthfully, because he was nuts enough to tell me his whole life story honestly, and he told me everything he ever did wrong too. lol)
I was trying to keep mum on having already discovered I loved him deeply and wanted to be his wife, but what you see of me here is what you get with me IRL too, so five days later, it slipped out. I was talking about where we'd put our dishes -- cupboard wise -- when we were married.
Two days later, we were talking about what town we wanted to live in, when it dawned on me he never asked me if I wanted to get married. He told me that when I talked about cupboards he went silent -- in shock. Then he went home, thought about it, and thought I was the one too, so went along with me. We were engaged that night. One week after our first date. (Four months after we became friends.) That was mid April. We were married in early October.
I have noticed that younger generations hold off longer and longer. I also see the divorce rate rising higher and higher. You either know this is the one, or end the relationship if you know this isn't the one, or you find yourself constantly questioning, for years on end if he is. If you don't know, he isn't! And, if you can live without him, he isn't!