As I mentioned earlier, they are on a fixed schedule. What you're seeing is the result of harvesting them at different rates. For example, there are 800 crops every harvest, and let's say that the crops appear every 4 hours. If you put all of your villagers to work harvesting them, they might be all gone in 30 minutes. You will have to wait for 3-1/2 hours for more crops. If you put only 3 villagers to work harvesting, it might take them 3 hours to harvest the 800 crops, and you'll have to wait only 1 hour.
Part of the strategy of Virtual Villagers is effectively managing the work assignments to balance population, food, tech, and building.