Standard Grade and Curb Height for ADA Compliant Shower Floor
https://www.ada.gov/regs2010/2010ADAStandards/2010ADAstandards.htm


  608.7 Thresholds. Thresholds in roll-in type shower compartments shall be 1/2 inch (13 mm) high maximum in accordance with 303. In transfer type shower compartments, thresholds 1/2 inch (13 mm) high maximum shall be beveled, rounded, or vertical.


Ramps(in general) are permitted to be 1:12 (though it is recommended that they be less steep), but areas that are considered wheelchair turning, transfer, parking etc. are limited to 1:48. Nothing specific to "shower floor grade" is in there, but depending on how it's intended to be used (or intended to be ABLE to be used) you could get the 1:48 limitation applied - it's still perfectly good for drainage (1/4" per foot.) 1:12 is not particularly comfortable for normal folks, and is far more than is required for plumbing drainage.