The crops are on a fixed cycle, which varies with the game speed setting. On normal speed, the crops arrive every 4 hours, regardless of how quickly your villagers harvest the crops. If your villagers harvest them in 3 hours, the farm will be empty for an hour; if they harvest them in one hour, you will be waiting 3 more hours for crops. One thing that can disrupt that cycle, however, is adjusting your computer's system clock (deliberately or inadvertently).

You will find the saga of another person here who was in a similar situation and the forum community offered a lot of help to get a successful game going.