How many years should American investment immigrants wait?

Recently, I also heard a few friends who applied for investment immigration in Redford 20 14, Shenzhen, saying that their I-526 has been approved, but they are all waiting for the scheduled face-to-face notice. People from the immigration company told them that it was difficult to predict the specific number of years because they didn't know what the immigration policy would be like. But it usually takes 3-4 years.