Prerequisites: Cropland Level 5
Here your grain is milled in order to produce flour. Based on its level your grain mill can increase your crop production by up to 25 percent.
The percentage added to your production is based on the gross production of your croplands. This means the production of your fields without any bonuses.
The level dependent attributes (such as construction costs, generated culture points and overall crop production increase) can be found here.
For the full table of construction times click this link for server with 1x speed and this link for server with 3x speed.