The crops are on a fixed schedule, but you don't see the time remaining until the next crops come in until they are all harvested. If you put all of your villagers to work harvesting crops and get them all in the food bin in 10 minutes, you will see a much longer time to wait than if you harvested the crops more slowly. If you let the game's time progress normally, you should easily be able to support 10-12 villagers on the crops alone while you work toward building up tech points.