/enums/target_frequency_time_unit.proto 

--- v18/enums/target_frequency_time_unit.proto  2025-08-05 14:36:09.000000000 +0000
+++ v19/enums/target_frequency_time_unit.proto  2025-08-05 14:36:16.000000000 +0000
@@ -39,5 +39,8 @@

     // Optimize bidding to reach Target Frequency in a week.
     WEEKLY = 2;
+
+    // Optimize bidding to reach Target Frequency in a month.
+    MONTHLY = 3;
   }
 }