Run function for set period of time

void setup() {
  // put your setup code here, to run once:
 pinMode(13, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:

  fastBlink();
  digitalWrite(13, LOW);
  delay(2500);
}

void fastBlink() {
  unsigned long interval = 2500;
  unsigned long startTime = millis();
  while (millis() - startTime <= interval) {
    // your code goes here...
    digitalWrite(13, !digitalRead(13));
    delay(50);
  }
}