It depends on how you define a relationship.
If you mean any kind of relationship, including platonic ones, while the duration often defines the experience, I've also known a few people with whom I've had an unfortunately brief but intense relationship. I'm sure it would have gotten even better over time. Death was usually what put an early end to the relationship. I was just telling Sacha (oops) Jason in PM about a friend of mine from Prince George in Vancouver, BC. I only knew him for a few years, but in that time we shared so much that it felt like I'd known him all my life. We visited each other often so this was a real life friend.
In terms of sexual relationships, the length enhanced the experience. I met Dick when I was 16 years old. We got to become best friends first although part of me always lusted for this handsome older man. When I was legal he let me seduce him. That really enhanced the experience. Eventually we were partners for 17 years. By the time he was killed we had gotten to a point where we could do silly things like finish each other's sentences when talking with other people. We could tell how the other felt based on some sort of energy or whatever you want to call it. No body language or words were needed. I even knew how he was feeling when we weren't together and I'd sometimes phone him to ask what's up.