Symptom: The rendering process is taking too long, or is taking longer than expected.
Issue: Many factors can affect the amount of time required to render an image, including image size, resolution, render quality settings, lighting, complex materials, and more.
Solution: See Rendering Best Practices. That section describes many of the factors that affect render time. It also provides recommendations and tips on how to balance the need for a quality image with the amount of time required to render it.