Box turtles lay eggs in the spring and early summer. Here in South Carolina some are known to lay in the fall. The eggs hatch in the fall and the hatchlings over winter in the nest or the eggs hatch in the spring.
A gravid female typically becomes very restless and constantly tries to escape. In my colonies it doesn't seem to matter how big the pen is.
When they finally decide to lay eggs (usually early morning or early evening), not much will distract them. It takes about an hour to dig a flask shaped nest. And just as long to bury it. The other night I watched for quite some time while a female eastern covered the nest. The movement of the hind legs reminded me of a baker kneading dough. When she was done, it was impossible to tell where she had dug.
This year I decided to try something new. I Got a small Rubbermaid storage bin and cut down one side about 2 inches. I punched some holes in the bottom and filled it with topsoil. I put it in a corner and made a ramp out of shale. The new nest box was an immediate hit.
Following are some pictures of egg laying. Click on the photos to view full size.