i have read books on this, and browse the internet looking for solid advice to this problem. you know people, i haven't found solid and consistent advice. there are many different schools of thought, that people offer: make a clean and pronounced break, or simply allow things to drift and do not say anything, or you can say something about the relationship when you two speak again. all of this advice is extremely conflicting and doesn't really allow for consistent advice.
some of the best advice i found was it good to let friendships go when the cost of maintaining the relationship became to high. for example, if you are the one that always must arrange you two to get together, and it bothers you, you should let the friendship go. or it may be something like you have to constantly keep each other in communication or the friendship will die, then you should let it go.
other advice i have seen is when that person isn't there to support you in times of need it is time to put that friendship on the back burner. friends should be there to support each other in time of need, and yet, when a person who was a friend may not be there for you during your important times of need, it may be time to let that person go.
i would like to say that it is time to let friendships go when you give more than you receive. i really feel relationships by nature, are often uneven, so that is a hard way to judge a relationship. once i heard a good explanation for friendships-- some friends were there for a lifetime, and others only for a season.