#include <Adafruit_NeoPixel.h>
#define PIN 5 // Neopixel pin
#define PIX 12 // Number of Neopixels
Adafruit_NeoPixel pixels = Adafruit_NeoPixel(PIX, PIN, NEO_GRB + NEO_KHZ800);

unsigned long fullTime = 20;

void setup() {
  Serial.begin(115200);
  pixels.begin(); // initialize a Neopixel object/instance on pin 3
  pixels.clear(); // clear any set pixels in the buffer
  pixels.show(); // display the pixel buffer
}

void loop() {
  fuel(fullTime, millis() / 1000); // "full" time and "now" time
}

void fuel(unsigned long whole, unsigned long part) {
  unsigned long value = PIX * part / whole;
  for (int i = 0; i < value; i++) {
    pixels.setPixelColor(i, pixels.Color(255, 255, 255));
    pixels.show();
  }
}