Physics say its infinity. But when things get distant the minute details gets mixed up.One can just figure out on a desert that a camel is coming a mile away but cant really make it out whether its has some stuff in its mouth and chewing it.

2.5 Million Light-years. The human eye can see an almost unlimited distance. Looking up into the sky on a very clear night, the Triangulum Galaxy can sometimes be seen. This is a distance of 3.14 million light years. The Andromeda Galaxy is also sometimes visible. This is a distance of 2.5 million light years. However, it must be pointed out that when we see these galaxies what we actually "see" is something at a somewhat lesser distance. What we "see" is light from the far away object rather than the object itself. What happens is that light coming from the object strikes our optic nerve. The optic nerve signals the brain. The brain then interprets this message and forms an image. The brighter the object, the farther away it can be from us and still be "seen". The star, Rigel, is many times brighter than another star, Alpha Centauri, yet Alpha Centauri is many times closer. According to Dr Brent Archinal of the Astrogeology Team of the US Geological Survey in Flagstaff, Arizona writing in the May, 1997 issue of Astronomy, the human eye can see about 2,500 stars in the clearest of night skies. But usually only about 1,500 to 2,000 stars can be seen due to weather, pollution, etc, and fewer still in night skies of cities due to the additional factor of city lights. Five planets can be seen from earth with the naked eye under the best of circumstances: Mercury, Venus, Mars, Jupiter, and Saturn.

On earth for pratical purposes given the curvature of the earth at or near sea level, the distance the eye can see on a clear unobstructed day is 16 miles, just another interpretation of distance possible.

the stars

Inwardly...1 and 1/2 feet...from the head to the heart, longest distance in the universe I might say. Lamentably, no magnification per se is possible, however moments of reflection, caring and of course, a touch of pain here and there in our daily lives, are helpful to bring about the really important things into focus.

2 million light years to the andromeda galaxy

5 billion billion billion light years away

In principle, there is no limit to how far you can see, but the farther away the objects are, the brighter they have to be for our eyes to be able to detect the light. So magnification really isn't the issue, it's light gathering ability. If our eyes were huge, say like pie plates, even if things didn't look any larger, we'd be gathering more light at night and able to see dimmer objects. The huge telescopes that you see in magazines don't necessarily have huge magnification, but they pick up an amount of light in proportion to the square of their diameter. Also, these telescopes use long exposure times in order to gather more light. Something our eyes don't do very well, but photographic film and electronic sensors are very good at.