How to calculate time needed for instruction ?

bit Flag_10mS=0, Flag_100mS=0, Flag_1Sec=0;	        // Timer Flags
unsigned char Count100mS=0, Count1Sec=0,Count1Min=0 ;// Time counters


/***************************************************
 *                    INTERRUPT SERVICE ROUTINE                      *
 ***************************************************/

void interrupt isr( void )
{

    if ( TMR1IF && TMR1IE )	// Timer1 overflow event occurred, set to 1mSec?
    {
        Flag_10mS = 1;
        TMR1L = 0xEF;		// Initialize Timer1 low byte
        TMR1H = 0xD8;		// initialize Timer1 high byte 1mSec
        TMR1IF=0;		// Clear TMR1 Interrupt Flag
        Count100mS++;	// counter for 100mS
         if ( Count100mS >= 10 )  // counter for 100mS
         {
              Flag_100mS = 1;
              Count100mS = 0 ;	// Reset counter
              Count1Sec++;
          }
          if ( Count1Sec >= 10 )	// Counter for 1 sec
         {
              Flag_1Sec = 1;
              Count1Sec = 0 ;	// Reset counter
         }
    } 
}
/******************************
 * Timer1 initialization code *
 ******************************/

void Init_Timer1( void )
{
    T1CON = 0x00;	// T1CON register setup
    TMR1L = 0xEF;	// Initialize Timer1 low byte
    TMR1H = 0xD8;	// initialize Timer1 high byte 10mSec
    TMR1IF = 0;	// Reset Timer1 Overflow H/W flag
    TMR1IE = 1;	// Enable Timer1 Overflow interrupt
    TMR1ON = 1;	// Start Timer1 counter
}

/***********************************************
 *                        INITIALIZE SYSTEM                          *
 ***********************************************/

void main(void)
{
Init_Timer1();	// Initialize Timer1
PEIE = 1;		// Enable Peripheral Interrupts
GIE = 1;		// Enable Global Interrupts

while(1)
{
// ************** 10mSec Function *****************
   if ( Flag_10mS )	// Functions to run at 10mSec
   {
       Flag_10mS = 0 ;
    ....
    }//End 10mSec

// ************** 100mSec Function *****************		
   if ( Flag_100mS )	// Functions to run at 10mSec
{
      Flag_100mS = 0;
      ....
}

// ************** 1Sec Function *****************
if ( Flag_1Sec )	// Functions to run at 1Sec
{
     Count1Min++;
     Flag_1Sec = 0 ;
     ...
}// End 1Sec

// ************** 1Min Function *****************
if ( Count1Min >= 60 )	// 1 Minute passed )
{
    Count1Min = 0 ;
   ...
}//End 1 Min
}//End while
}//End main