volatile uint32_t old_millis;
int seconds;

void setup() {
  Serial.begin(9600);
  Serial.print(" ");
  pinMode(LED_BUILTIN, OUTPUT);
  digitalWrite(LED_BUILTIN, LOW);
}


void loop() {
  if (Serial.available() > 0) {
    seconds = Serial.parseInt();
    old_millis = millis();
    Serial.read();    // вычитать буфер от мусора
  }
  if (millis() - old_millis < 1000UL * seconds) {
    digitalWrite(LED_BUILTIN, HIGH);
  } else {
    digitalWrite(LED_BUILTIN, LOW);
  }
  delay(10);
}