PInvoke: When is the memory allocated by the CLR marshaller freed?

I am using PInvoke to pass the following class to unmanaged C++ when the app starts up. All the values are passed in correctly as I step through the code. Inside the called C++ method I save a the ptr to the passed in struct.

  [StructLayout(LayoutKind.Sequential)]
    public class Config
    {
        public int Speed;

        [MarshalAs(UnmanagedType.ByValArray, ArraySubType = UnmanagedType.LPStr, SizeConst = 2)]   
        public string[] Settings = new string[2];
    }

Matching C++ struct

struct config 
{
    int            Speed;
    char*        Settings[2];

  }

This is the method in C++ that saves ptr to the class that is passed in

Config *m_config;

void SetConfig(Config *config)

{

    m_config = config;

}

Question: if the marshaller allocated memory for the C++ struct at the time of marshaling when does it free the memory?

If I save the ptr to the passed in struct, will that ptr be invalid as soon as the function returns to C#?

Is there a way to tell the marshaller to NOT deallocate the memory it allocated during marshaling?

Thanks for any info or suggestions.

Mitch