Depth of demineralization for dentin is:
In enamel, the resin tags are approximately 6 µm in diameter and 10 to 20 µm in length and lead to micromechanical interlocking.
Phillips 12th edition page 260
Dentin is demineralized by up to approximately 7.5 µm depending on the type of acid, application time, and concentration.

Sturdevant 7th edition page 144
10 micron
7.5 micron
6 micron
20 micron