There is no specific value, but it can be in range!! Testa thickness is so varied from one species to another! Even within the same species several factors can affect testa thickness like growing conditions.
Check this out:
"Testal seeds can be divided into exo-, meso- and endotestal seeds, according to the position of that main layer. In exotestal seeds that layer differentiates from the outer epidermis, in mesotestal seeds from the mesophyll, and in endotestal seeds from the inner epidermis".
All morphological characteristics of seeds such as size, seed shape, seed coat surface pattern, endosperm structure, and of course seed testa thickness may vary widely among plant species and even in a same species....