Rose...
well, having never been in a relationship, I'll speak from my experience of seeing people around me.
My mum and dad. They have been married for 24 years and have never had what you'd call a love-based relationship. They could never work it out. They continued just for our (me and my sister's) sake. They have been separated for the past 3 years (not divorced).
The point is, it depends on relation to relation and person to person. One can have an eually intense relationship for a shorter period of time. The only difference would be the stability. The assurance that your S/O would be there for you, no matter what.
As I said in another thread, Count not the moments you have spent in your life with him/her. Count the life that was in each moment that you spent with him/her.
I'm hope I'm making sense. I just had a wonderful, but tiring, day with my best friend. Sigh!