MyoWare_Test.ino

const int SENSOR_PIN = 0;
const int CYCLE_DURATION = 250;

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  int reading = analogRead(SENSOR_PIN);
  int duration = map(reading, 0, 1023, 0, CYCLE_DURATION);
  digitalWrite(LED_BUILTIN, HIGH);
  delay(duration);
  digitalWrite(LED_BUILTIN, LOW);
  delay(CYCLE_DURATION - duration);
}

To see a version of this code with some Serial output of readings in it, follow this link.