10mm min to 15mm max, less (say 5mm) if your brickes are bang-on. The problem is that, at least in my experience brickies don't always give you the opening you require, i.e. straight and plumb. This is especially true if they are cutting out existing masonry with a Stihl saw to create an opening. In stud walling and MF you can pretty much guarantee that it will be right and work to a 5mm tolerance. There's also the issue of fire doors where you may need about 8 to 10mm gap to form an effective fire barrier with mineral wool (foam alone is no longer permitted AFAIK). BTW 10mm packers are readily available
Generally 3 fixings on the hinge side (ideally beneath the hinges, for which the hinge recesses need to be pre-cut and the screw holes piloted) and the same on the other side (top, just above/below the keep and near the bottom). Double-up horizontally if the door casing/lining is exceptionally wide, drilling and counterboring any visible screws so that the screw holes can be pelleted and trimmed back when you are done. Fire doors give you a bit more leeway if the masonry is ropey as the drillings can be done in the intumescent strip grooves (where they won't be seen after the strips ar installed)
