From the Airbus publication "Getting to Grips with Aircraft Performance", page 145:
In order to maintain a minimum margin against buffeting and ensure good aircraft maneuverability, it is necessary to determine an acceptable load factor limit below which buffeting shall never occur. This load factor limit is generally fixed to 1.3. This value is an operating limitation, but not a regulatory one. The corresponding altitude is called the “1.3g buffet limited altitude” or “buffet ceiling”.
So 1.3 is a "normal minimum" but I'm sure someone has a counterexample out there somewhere.
The example that follows indicates a case where the selected cruise Mach could be in a range where the buffet margin is 1.3 or greater. There will be cases where the optimum lies in the middle of that range, and so the margin would be higher than 1.3.