If the purpose of the scene is to show a developing relationship, or the depth of a relationship, then I don't see it as useless. The sort of story you're talking about revolves around relationships, and so that's part of the story.
Showing the same thing multiple times may be a waste and drag the story down, but if it's being used to develop the characters, then it has a purpose and there's nothing wrong with it being there. How long the scene should be is a judgement thing -- it should be long enough to show what you want to show, without being filled with so much 'stuff' that the reader can't wait to get to the end of it.
In short, there really aren't any guidelines. It's up to the author to decide what they want the scene to show or do, which might be advancing a plot, or maybe developing the characters, or both, and then to write a scene that meets that goal without making the reader fall asleep