place the code shown below inside a timer interrupt handler configured to fire at 500 microseconds.
if the key(s)is kept pressed for consecutive five increments then the program flow will take another path
resulting in rapid value increment of any desired variable.and as soon as the program see any keyup it will
resume to old slow value increment mode.HOPE THIS WILL HELP.....


/*---------here starts the code for incrementing value of variable X --------------------------------------*/

if(button_PLUS == 0) {
COUNT_PLUS++; }else {
COUNT_PLUS = 0;
RAPID_SCAN_COUNT_PLUS = 0;
RAPID_SCAN_FLAG_PLUS = 0;
KEY_SCAN_RATE_PLUS = 80;
}
if(COUNT_PLUS > key_scan_rate_PLUS) {
//update lcd value here or can be X++
COUNT_PLUS = 0;
if(RAPID_SCAN_FLAG_PLUS == 0)
RAPID_SCAN_COUNT_PLUS++;
}
if(RAPID_SCAN_FLAG_PLUS == 0) {

if(RAPID_SCAN_COUNT_PLUS > 5) {
RAPID_SCAN_FLAG_PLUS = 1;
KEY_SCAN_RATE_PLUS = 10; }
}


/*---------here ends the code for incrementing value of variable X --------------------------------------*/


/*---------here starts the code for decrementing value of variable X --------------------------------------*/

if(button_MINUS == 0) {
COUNT_MINUS++; }else {
COUNT_MINUS = 0;
RAPID_SCAN_COUNT_MINUS = 0;
RAPID_SCAN_FLAG_MINUS = 0;
KEY_SCAN_RATE_MINUS = 80;
}
if(COUNT_MINUS > key_scan_rate_MINUS) {
//update lcd value here or can be X--
COUNT_MINUS = 0;
if(RAPID_SCAN_FLAG_MINUS == 0)
RAPID_SCAN_COUNT_MINUS++;
}
if(RAPID_SCAN_FLAG_MINUS == 0) {

if(RAPID_SCAN_COUNT_MINUS > 5) {
RAPID_SCAN_FLAG_MINUS = 1;
KEY_SCAN_RATE_MINUS = 10; }
}


/*---------here ends the code for decrementing value of variable X --------------------------------------*/

Regards
SAIF ULLAH KHALID.