Probably about 5 months. The last month of fall and the first month of spring (around here, that is Novemeber, December, January, February, and March it would be empty). Then in Spring it gets plowed up and the plants start growing in Summer. For simutrans purposes, the winter scene would be dry grass or snow; the spring scene would be plowed, brown soil with a few green seedlings; the summer scene would be green plants; and the fall scene would be the green plants with a heavy layer of white static on top.
I wish I had time to do this for you, but I haven't even started on that quarry I was going to do yet.