Based on personal experience, the time will obviously vary between age of the individuals. I think the key factor is how far along in the relationship are you? By this I do not mean you "think" you love someone you ve dated for the past X amount of years, but what happened during that time makes you want to stay together.
I m only 24, my fiancée 23 and we have been through some pretty tough times. For most it may be arguments, money, and differences in opinions, but her and I share a lot of the same parenting, social, and economic views. But we were initially going to wait until we were 27-28 years old before we thought about marriage, but something happened that made this process go a little quicker.
At the risk of sounding selfish and eccentric, I do not mean to when I say this. But for us, I was involved in a serious motor vehicle accident. I was hospitalised for a month where I was in a coma for a week, and I have an acquired brain injury because of this. During the beginning stage of when I was first emitted into the hospital, I was unconscious and had to undergo brain surgery. My girlfriend at the time was there first, signed the papers, and because of her I am still here living today.
She stuck by my side the entire time I was in a coma, left her job to be by my side, and her and her dad started the insurance process for me (as his job is based around law and legal issues). She basically did what a priest would ask you on your wedding day, which in this case was "will you stick by them, through sickness and health?". She did.
So for us, I knew she was the girl I wanted to marry before the accident, but seeing everything AFTER I knew I had to make it happen. We are still young, and only 3.5 years into our relationship, we met in college, and we share a lot of the same views. Yes our age and length of relationship is shorter than I would say is needed for our age, but in my personal experience if you have all that stuff and then add in the wedding vows involuntarily and STILL stay by each others side, then THAT is what means you are ready.
So yes, as you get older the wait time to get married is short, and as you are younger the wait time should be longer. But what happens in those years in the relationship is also a factor in my opinion. If you have lots of ups and downs and it is a healthy relationship otherwise, I believe the only thing that should come in to consideration is how YOU feel when you should pop the question (and obviously knowing she or he is wanting that too!)