According to ARM, the max length of an Unbounded_String is fixed to Natural'Last, that is: 2147483647
However, this example shows an error (Storage_Error) when the Unbounded_String try to contain more than 2048000 characters.
This problem is due to some limitation of the implementation of Get_Line, that is recursive. On UNIX OS (Linux, MacOS) the max recursivity is defined with ulimit -s.