A friend of mine told me that she read somewhere (this isn't quite the greatest citation ever) that a relationship needs to go through a "forward" or "progressive" change about every 4 yrs. So for a lot of couples, they might move in together after about 4yrs, get married after about 4yrs, have a child, etc. It gets pretty tough the longer you're together, and so relationships take work, esp after a couple no longer has the kids in common (Most divorces occur in the first 7 yrs of marriage, or after about 16yrs)
The other thing that I've heard from a psychologist is that it takes about 3 yrs to form a truly intimate bond/trust with someone.
So I think if a)you're really not planning to move the relationship forward, or b)you may have formed the bond, but don't feel it's enough or have had enough, then don't prolong this! You can break up and still be friends (after a little bit of time). Dragging this out will make you resentful. And give yourselves some time away from each other. It's too easy to hook up with someone you're comfortable with, just b/c you're comfortable with them.