Depending on the coloration of the material in of the texture and how rough it is, several feet to hundreds of yards/meters away.

But the reason brutalism is easier to see far away is more about the solid rectangular shapes and style than the texture of the material. If you made a concrete version of a building normallly made of stone with lots of fine details like a cathedral it wouldn’t be considered brutalism just because it was all concrete.

Brutalism is also about showing the materials used for the structure rather than using facades. (But I don’t think anyone is saying rough rock coverings are brutalist lol)