It depends on how big it is. But if you can pick it up and it feels like "nothing" it's too thin. If you pick it up and it feels like there is something in there, it's not too thin, If, as was said, they can't close their shells, they are too fat!
I know that sounds vague, but as I said, the "normal" weight can vary, depending on size, gender, and species.
I have two adult boxies. The ornate female weighs 400k, the male eastern weighs 325k. (So both are just under a pound.)
Hope that helps.
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0.1.0 ornate box turtle
1.0.0 eastern box turtle
1.0.0 Yellow belly slider
0.1.0 Red belly cooter
0.1.0 Australian shepard
1.11.0 chickens
1.0.0 Dutch(rabbit)
3.2.0 children (do I still count the married ones?)
1.0.0 husband