At vero eos et accusamus et iusto odio dignissimos ducimus qui blanditiis praesentium voluptatum deleniti atque corrupti quos dolores et quas molestias excepturi sint occaecati cupiditate non provident, similique sunt in culpa qui officia deserunt mollitia animi, id est laborum et dolorum fuga.
Et harum quidem rerum facilis est et expedita distinctio. Nam libero tempore, cum soluta nobis est eligendi optio cumque nihil impedit quo minus id quod maxime placeat facere possimus, omnis voluptas assumenda est, omnis dolor repellendus.
Itaque earum rerum hic tenetur a sapiente delectus, ut aut reiciendis voluptatibus maiores alias consequatur aut perferendis doloribus asperiores repellat.
Not the answer you are looking for? Search for more explanations.
Assuming the cream is colder than room temperature, say at 5 deg. C and possibly maintained at that temperature during the 15 minutes, and the coffee is hot (say at 95 deg.C), then I suggest the following:
Assuming Newton's law of (forced convection) cooling applies, cooling rate is proportional to the difference between the room temperature and the liquid temperature.
If the cream has the same temperature now and in 15 minutes, then we're better off adding now, to cool the coffee down to 90 degrees, say, to reduce cooling loss in the next 15 minutes.
If the cream is not refrigerated, it "may be" better to add it later, because then the cream gets warmed up to room temperature before it cools the coffee. You may need to do a little calorimetric calculations to confirm which is a better choice.