Recomended minimum disc thickness is 28mm front and 20.2mm rear for Brembo brakes.
Pads should be changed at around minimum of 2/4 mm however pads should also be changed if they start to break up or crack and you would have to remove the pad to determine this.
Also on the rear the inner pad wears down much quicker than the outside one, so you will have to remove the wheel for a proper inspection.
Alex