const int ledPin = 13;

unsigned long previousMillis;
unsigned long interval = 200UL;   // not a constant, it will be changed

void setup() 
{
  pinMode( ledPin, OUTPUT);
}

void loop() 
{
  unsigned long currentMillis = millis();

  if( currentMillis - previousMillis >= interval) 
  {
    previousMillis = currentMillis;
    interval = random( 100, 800);   // set the new interval

    // toggle
    digitalWrite( ledPin, digitalRead( ledPin) == HIGH ? LOW : HIGH);
  }
}