The final render took about 30 hours at >5k samples per pixel. While prototyping I typically used a resolution of 1000x563 (low-res 16:9) and used the incremental renderer in the editor, meaning that the first images would come back after a few seconds. This was usually enough to figure out whether a composition would work or not.
To setup and tweak the lighting, I needed more converged renders, and that could take anywhere from 15 minutes to several hours. Most of the museum converged pretty quickly, making it easier to set up the sun and these sorts of things, but the planet would stay noisy for a very long time. Multiple scattering in the atmosphere has enormous variance and tends to just spray bright blue dots throughout the scene. Any change to the parameters of the planet, atmosphere or clouds would require a few hours of rendering before I was able to check whether the change was for the better or worse.
Not the most efficient feedback loop, but I had other final projects to work on while renders were running, so it wasn't too bad.