Does long-standing sound right to native speakers in this context? Example: Your long-standing effort has paid off. Does this sound idiomatic?
Given the fact that I would personally use long-lasting efforts, replacing standing with lasting and making it plural, I checked something objective and queried Google Books Ngram Viewer.
According to it, while the plural form is the more common, so, too, is long-standing, at least in printed form.
Per this, your use of long-standing effort (or long-standing efforts) is just fine.