An example showing how to put a ESP8266 device into Deep-sleep mode

esp8266-deep-sleep.ino

/**
 * An example showing how to put ESP8266 into Deep-sleep mode
 */
 
void setup() {
  Serial.begin(115200);
  Serial.setTimeout(2000);

  // Wait for serial to initialize.
  while(!Serial) { }
  
  Serial.println("I'm awake.");

  Serial.println("Going into deep sleep for 20 seconds");
  ESP.deepSleep(20e6); // 20e6 is 20 microseconds
}

void loop() {
}

I also think it meant to say 20 seconds, my test seems to show it is 20 seconds.

As the seconds value (20) is mentioned in the line above I suspect it was meant to say
ESP.deepSleep(20e6); // 20e6 is 20000000 microseconds

Just in case the reader is not familiar with Exponential Notation (It simplifies numbers by getting rid of zeros, and making math easier)