Doors and Windows - Ground grade below window

We are going to have two windows in a concrete block wall that will have back fill (dirt) then sod placed on top of that dirt just below these two windows.
My question is how close can I backfill the dirt to the bottom of the window ?
Is there an industry standard ?
I don't want the grass so close that I cant mow or weed trim without getting debris all over the glass. On the other hand I want to backfill as much as I can.
What is a good close but safe distance ?
Thanks

Question...are you putting an office or bedroom in the basement? If so the window has to satisfy egress requirements for emergency exit. In any case, you would usually put a window well (galvanized steel or plastic) if the window is below grade. Once attached to the foundation you can backfill with dirt/gravel and buy a cover to protect the glass from debris and water.