my_box->model->function = i_am_fly;//moves the model on the x-axe

typedef struct
{
   var lifetime;
   ENTITY* model;
} box; 


function i_am_fly(box* ThisBox)
{
   ThisBox.lifetime -= 1;
   if (ThisBox.lifetime < 1)
   {
      ent_remove(ThisBox.model);
      //	Add the following line ALSO if you want to remove BOX struct too
      //ptr_remove(ThisBox);	
   }
}

function new_box()
{
   box my_box;
   my_box.model = ent_create("a.mdl",vector(0,0,0),NULL);
   //my_box.function = i_am_fly();   //<<<<<< replaced lower down
   /* some 3d object set up stuff */
   my_box.lifetime = 100;
   c_setminmax(my_box.model);
   wait(1);
   /* some object physics set up stuff */
   i_am_fly(my_box);              //<<<<<<<<<< only new line in new_box() 
   num_box=num_box+1;
   ... 

#include <acknex.h>
#include <default.c>

ENTITY* FLOOR;
var ypos;
var hello;
var angle1;
VECTOR box_rotation;
VECTOR centre = vector(0,0,0);

var num_box;
var new_ball_delay;
var iveruntheIamFlyfunk;

typedef struct
{
	var lifetime;
	ENTITY* model;
} box; 

function i_am_fly(box* Thisbox)
{
	iveruntheIamFlyfunk += iveruntheIamFlyfunk;
	Thisbox.lifetime -= 1;
	if (Thisbox.lifetime < 1)
	{
		ent_remove(Thisbox.model);
		ptr_remove(Thisbox);
	}
}
  
function new_box()
{
	ypos = 10;
	while (ypos < 450) 
	{
		ypos = (ypos + 25);
		angle1 = 0;
		while (angle1 < 360)
		{
		angle1 += 360;
		box my_box;
                
                /*box object setup stuff */

		c_setminmax(my_box.model);
		wait(1);

		/*box physics setup stuff */

		num_box=num_box+1;
		i_am_fly(my_box);
	}
	}
	wait(1);
}

function i_am_fly(box* Thisbox)
{
	while(Thisbox.lifetime>0)
	{
		wait(1);			//wait 1 frame
		Thisbox.lifetime -= 1;	//reduce counter
	}
	ent_remove(Thisbox.model);		//remove timed-out model
	ptr_remove(Thisbox);		//remove empty box
}

/// Assume in new_box you said 
...
my_model = ent_create("cube_small.x",vector(0,0,0),NULL);
my_model.skill25 = 100;
i_am_Dying(My_Model);
...

function i_am_dying(ENTITY* ThisModel)
{
	while(ThisModel.skill25>0)
	{
		wait(1);			//wait 1 frame
		ThisModel.skill25 -= 1;	//reduce counter
	}
	ent_remove(ThisModel);		//remove timed-out model
}