That would depend on the cemetery. The graveyard my family lie in had graves over 150 years old. The older coffins were of plain wood, and the land is bedrock with soil not so deep. Many coffins only had a couple feet of soil over them to begin with.
I had the un-nerving experience as a teen of stepping too close to one of the older graves, which are often identified by their sunken appearance. My leg went into the grave halfway up my shin, and I don't know (nor care to) how much of what I felt and heard by way of crunchings and squashings was down to roots and earth or rotted wood and brittle bones. (Again, I am so sorry, Great-great-great-aunt Margaret!)
The moral of that story is: even at your standard six feet, soil erodes, wood rots, empty spaces fill in eventually. It's best to take care walking near (and especially on) old graves, and gardening atop them, especially if using a plough or tiller, ought to be approached with delicacy.