Mains to low voltage seperation between FR4 PCB layers
This is information you get from the datasheet of the prepreg/laminate you use. Example here from Isola:
http://www.isola-group.com/wp-content/uploads/2012/09/IS420-Lead%C2%ADfree-Laminate-and-Prepreg-Data-Sheet-Isola.pdf

This specific material datasheet show something like 30 kV isolation per mm material layer to layer.

If you rely on this, make sure the material is fully specified in the PCB fab drawing.

Also note you can pretty much count on the laminate thickness, but the Cu traces will sink into the prepreg effectively reducing the resulting layer to layer distance.