I have heavy clay and have dug up about 6 in so far.
When Adding Compost to Soil How Deep Should I Go?
Many gardeners would say you could never add enough! Seriously, it depends on what you are trying to grow. 6" of loose soil full of compost will grow great annuals and azaleas (in a shady area). Root crops of carrots and potatoes will do best in 12" ammended soil. Native trees to your area would do just fine in the clay. Sorry to not be more specific, but again, it depends on the 'crop'.