The first part of this verse appears in the previous verse in the KJV. We put it here because it appears here in the sources we use today. The "sixth hour" would be the middle of the twelve "hours" of daylight, so noon. At the ninth hour, the workday would be three-quarters gone, say 3PM. So the NIV translations give reader an idea, but they do not represent the words at all. 

Notice that every time the vineyard owner goes out, there are more people in the market, even though he hires all he sees on every visit. Where were these people earlier? In modern times, we organize our lives around our jobs for employers, but in agricultural societies, jobs are more task-oriented. Many of these people may have spent most of the day tending their own land or flocks. Others may have been selling in the market until they ran out of produce. Others may have found work for others that didn't last the full day. After their work was done, people gathered in the marketplace, not only to find more work but to visit with other people and socialized.