Brings up an idea to me. The preview. There we have a complete rendering so to say. Maybe the time the preview needs could be taken for the render estimation? The idea came into my mind because the only way to estimate more accurate is to "pre-render" the image in a low format first and then multiply that with some factors (pixel size, anti-aliasing quality etc etc). I can imagine, it could be possible.

One day your ship comes in - but you're at the airport.