An average interview would be somewhere between half an hour and an hour. Group interviews are another thing. They are less likely to get at very specific individual issues, but they're good at getting a broad understanding of how different groups feel and sort of more to the collective issues.
I always try to have focus groups for female employees, in particular, who are often in the minority in a mining context. That's a good way for women to try to surface issues. Those tend to last a bit longer, an hour and a half.
Interviews with managers can be quite long, two to three hours, because I will actually go through their policies, procedures and so on. It's often with managers that I will go and inspect different areas of the work site and so on.
Interviews with government officials tend to be about an hour and a half. With the government, particularly the ministry of labour and human welfare, I had several at increasingly high levels.