<p>I am printing on a non-heated bed right now, but the question also applies to heated building plates. </p>

<p>How often should you replace the glue layers that's supposed to be applied before printing? Some say you can do up to a few prints, such as in this <a href="https://ultimaker.com/en/community/19056-glue-stick-or" rel="noreferrer">forum</a>, while others say to replace it every print. What is the correct approach?</p>

As noted in the answer to the other question you asked, the Flux Delta steel plate bed will handle multiple layers of glue. The determining factor regarding this particular printer and specific glue is how many ripples, bumps and/or lines you are willing to tolerate on the first layer of your prints.
You'll notice that a print made with a couple layers of glue, freshly applied, will have a relatively smooth surface. Peel off the model, apply glue over the now-cleared areas, and you've created a slightly-less-than-smooth surface for your next model.
I've found that I can apply six to ten layers before the ripples become objectionable.