Very good question. I've dug or am digging my fifth shelter in less than 4 months and I generally let my back tell me when to stop. Two of the shelters have sand to a depth of over three feet and the labor involved vs the return wasn't there at that level. It seems the deeper I go, the less I find and the artifacts aren't necessary older as I go deeper. The Kirk that I found yesterday was retrieved at a mere three inches. I will say this, I continue to dig methodically until I run out of the charcoal and ash layer of sand, whether it be 5 inches or 5 feet. I seldom find anything below that. The sand usually turns yellow or red and I stop. Everyone keeps telling me to dig til I hit bottom but it's hard to stay with it for two or three days when you're not finding even the smallest of flint flakes, especially when there's another undug shelter right around the corner. If I run 15 or 20 sifter loads with no flint, it's time to relocate within the shelter or altogether.
I think the river made shelters or shelters close to water supplies (flooding) generally might have more sediment in them and would require the 10 to 12 feet of digging that we hear about sometimes. The shelters in my region are high and dry and seldom will fallen leaves make it past or just past the drip line, nice....no decay.
I hope others will reply to this, I've been curious what others do as well.
