Time has nothing to do with it.
People tend to "RELAX" once they have landed someone in their lives. They don't put in the same effort they did when they were trying to "win" them over.
In any relationship we are either growing together or growing apart, nurturing or neglecting. When one person stops caring whether or not the other person is happy the relationship is coming to an end. It's also important that the unahppy person speaks up earlier rather than later.
Both people have to be willing to keep the garden of love alive!
(It's easier to maintain a fire than it is to reignite a spark!)
Once you let things slide too far it's nearly impossible to get back on track. Time is not the enemy, being lackadaisical is.