The five minute rule mentioned below is a little strict for my taste, ten or fifteen minutes sounds about right. If the “friend” notices that you weren’t there, didn’t wait, and asks you can blame yourself, “It drives me crazy when people don’t respect my time enough to be on time or at least call. I’m sorry, I get so angry that I fume inside the whole time and really get to resent you, and I I don’t want to resent you, I love you as a friend”.