FOR loop not working with my ultrasonic sensor hc sr04

Hi everyone.well I am very new to world of arduino.So hoping good solution for my problem
I am trying to work on water level sensing system using arduino with ping sensor sensor hc sr04.In my program I want to measure water level after every 4 mins with around 20 sets of level readings at the end of every 4 min with delay of 3secs.for this I inserted the for loop which seems to be ok but unable to produce any output on screen.while the system is working fine and giving out without FOR loop for particular time delay.
please suggest me for any error in my program or any other alternative method to work out for my code.
Here is my code

#include <NewPing.h>
#define TRIGGER_PIN 3
#define ECHO_PIN 2
#define MAX_DISTANCE 300

NewPing sonar(TRIGGER_PIN, ECHO_PIN, MAX_DISTANCE);
int level=0;

void setup() {
Serial.begin(9600);
Serial.println(“CLEARDATA”);
Serial.println (“LABEL,Time,level”);
}

void loop() {
delay(240000);
for ( int n=0;n>=20;n++){
int uS = sonar.ping();
level = uS / US_ROUNDTRIP_CM;
Serial.print(“DATA,TIME,”);
Serial.print (level);
Serial.println(" , ");
delay (3000);
}

}

 for ( int n=0;n>=20;n++)

Starting with n equal to zero, while n is greater than or equal to 20, do this and then increment n. How many times will this loop iterate?