It depends on which motorcycle you have, on some 'older' big bikes the disc thickness is sometimes stamped between the mounting holes. For sure you find the minimum disc thickness specified in the service / work shop manual of your motorcycle.
In general, manufacturers specify that the disc should be checked radially for any variance in thickness across its surface; a variation will cause brake judder and the discs should be replaced.
For the small-engine-sized (100 up to 125cc) scooters and motorcycles Honda standard is 3.3 or 3.7mm (0.13 or 0.15 inch) brake disc thickness, the brake disc needs replacement if below 3.0mm (or 0.12 inch).
The Honda service manual has service limit for brake disc warping of 0.30mm (or 0.012 inch).
By: news (offline) Wednesday, 22 June 2016 @ 11:18 AM ICT
Quote by: Allan
My motorcycle is a Honda CBR600R
For the Honda CBR600R the minimum permissible disc thickness is 3.5mm. The disc is standard 4.4 to 4.6mm thick. If you measure the disc make sure you only measure the area where the brake pads touch. Measure the disc about 2mm from the edge to get a good measure....