7" is about right.

turn plows are designed to plow a depth of half there width. (an 18" wide plow has a nominal cutting depth of 9", 16-8 14-7 and so on)

its relatively easy to make them cut less depth, you can simply raise the 3pt hitch and make shure the plow still sits level by adjusting the top link.

but to cut deeper is a problem, the dirt really doesnt have a place to get out of its way so it mostly builds up in front and causes you to either stop moving forward or the plow to ride up out of the ground.

a middle buster will get you deeper but stock ones are generally only good for about a foot. you can custom one to bust up the ground down to 18" but it wont do much to clear the dirt out of the trench. still going to be hand digging for that part.

if that doesnt sound like fun rent a trencher or backhoe