You probably already have a good answer to that question based on the time you talk to clients. Still, just how much time do they spend looking at each image? How about each part of the house? Can you put a solid number down, more than just a gut reaction?
Believe it or not, somebody has done the research on this. And the source is reputable: The Wall Street Journal. Take a look at the article with the research done on this theme. It will help you understand your clients better.