Thickness for polyimide stiffener in flex pcb
There is no standard value but the typical answer I receive from chinese fabs like Itead and Elecrow is 0.3mm for a polymide stiffener. 

Specifically from the Elecrow team it was suggested to me that if I insisted the stiffener thickness to be 0.6mm I should choose Rigid (FR4) material but normally the thickness is 0.3mm.

Now, when it comes to stiffener thickness for FPC cables (underneath the gold contacts) most datasheets suggest a thickness of 0.2mm for the polymide. (see here  and here  for 2 examples)