How should Isopropyl Alcohol be used as an AvGas Fuel System Icing Inhibitor?
FAA Advisory Circular AC No: 23.1521-1B deals with use of IPA for Part 23 aircraft. It states:


  ASTM D 910, Standard Specification for Aviation Gasolines, allows the use of isopropyl alcohol conforming to the requirements of ASTM D 4171, specifications for Fuel System Icing Inhibitor, as a fuel system icing inhibitor. Accordingly, isopropyl alcohol conforming to ASTM D 4171 may be used in concentrations up to 1 percent by volume, to benefit safety, as an icing inhibitor in automobile gasoline.


However, it is left to the aircraft manufacturers (according to ASTM 910 too) to determine whether or not to use IPA as an additive. For example, in a letter regarding Lycoming engines in certain of their aircraft, Cessna says,


  Isopropyl alcohol in amounts not to exceed 1% by volume can be added only to aviation fuel  (not  automotive  fuel)  to  prevent  ice  formation  in  fuel  lines  and  tanks.  Although approved  for  use  in  Lycoming  engines,  do  not  use  isopropyl  alcohol  in  the  aircraft  fuel systems unless approved by the aircraft manufacturer.


Note that Cessna allows the addition of IPA only to aviation fuel for the applicable aircraft. In UK, the MoD Defense Standard 91-90 again leaves the matter to the individual manufacturers, saying,


  The concentration of Isopropyl Alcohol shall be recommended by the aircraft
  manufacturer at the time of delivery to the purchaser and typically not exceeding 1% v/v.


Though almost all these cap the IPA at 1%, the best way is to refer to the individual aircraft's appropriate manual as it would have the necessary details. But generally, I think that manufacturers (of large aircraft, atleast) prefer not using IPA these days due to environmental concerns.