Its not a matter of how far it can be zeroed, but a matter of how far it can be effective. A red dot is great for close targets and fast target acquisition. At distant ranges it can be adequate, but don't count on any type of precision. Even the smaller dots will cover the entire black area of a standard 100 yard target at 100 yards. I use a red dot on an AR for 3 gun competitions. I use it because I have to hit targets only as far as 100-150 yards. Those targets are about a 12 inch square, and it doesn't matter where I hit it, just that I do.
If you are looking for somewhat precise shooting, you are better suited with a scope. If you want to acquire larger targets quick, and not care where you hit them, go for the red dot.
-george