Minecraft wiki entry for Crop_farming, associated text: The worst case would be two crops diagonally adjacent on dry farmland (all other surrounding blocks being non-farmland) which has a growth probability of 1⁄23, about 4%.
The average rate of production per wheat crop can be found by the expression (6.591 x Growth Probability) wheat per hour, assuming the crops are harvested as soon as they are fully mature.