Here the flour produced in your mill is used to bake bread. Combined with the grain mill an increase in crop production of up to 50% can be achieved.
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.