#include <Adafruit_NeoPixel.h>

#define PIN 6
#define NUMPIXELS 15

Adafruit_NeoPixel strip(NUMPIXELS, PIN, NEO_GRB + NEO_KHZ800);

unsigned long startTime;
const unsigned long duration = 600000; // 10 minutes

void setup() {
  strip.begin();
  strip.clear();
  strip.show();

  startTime = millis();
}

void loop() {
  unsigned long elapsed = millis() - startTime;

  int ledsToLight = (elapsed * NUMPIXELS) / duration;

  if (ledsToLight > NUMPIXELS) {
    ledsToLight = NUMPIXELS;
  }

  strip.clear();

  for (int i = 0; i < ledsToLight; i++) {
    strip.setPixelColor(i, strip.Color(0, 255, 0));
  }

  strip.show();

  delay(500);
}