Roy Harris was a good friend of mine, and a great role model as a performer, though I agree with Sandman that he spent decades honing his skills, and an inexperienced performer would find his approach hard to emulate. Actually I remember him providing informative and amusing song intros, though he could also launch into a song without preamble when he felt like it, or even start the first song as he walked from his seat to the stage.

On the original question, when facing a folk club or similar 'listening audience', I would budget for an average of 5 minutes per song, including intro, unless I was planning a ballad-heavey set...