If we assume that there is nothing except the sun in the universe then how far the light can travel ??
Using your scenario, and lets ignore dark energy expansion and red-shifting at greater distances,. There would be no distance where the sun will disappear. That might seem counter intuitive but it's true.
On Earth, we receive about 10^17 photons per square CM. (source). Using the inverse square rule, to receive 10 photons per second on a pair of human eyes, that distance would be 100 million AU - give or take, or about 1,580 light years. We probably can't see just 10 photons, but in pitch black, with eyes adjusted to the dark, maybe we could. That's about the distance where the sun would give of steady but very very dim light.
Further than that, what happens has more to do with the quantum nature of light than the brightness and distance. As you double the distance you see 1/4th the frequency of photons. At 5,000 light years you see 1 photon per second, basically a very dim blinking light, but just as bright as it is at 1,580 light years, just appearing less often. Double the distance again and it flashes once every 4 seconds. There's no distance where you stop seeing the sun, you just see it less and less often, but it never appears dimmer than 1 photon.
Well, one reckons that light from the sun will travel to the end of our universe, if in fact it has an end. One way to calculate the actual distance to the end of our universe is as follows: (1) assume that our universe is a very large "black hole" ----- understanding that a "black hole" is simply a volume of space from which light cannot escape. (2) use Schwartzschild's famous formula for the radius of a "black hole":
R = 2GM/c.c where "M" is mass of universe. If one use numeric value M = 1.58E58 grams (following Sternglass), then one calculates that sun-light will reach the end of our universe after travelling approximately 2.35E30 cm.