In [this question](https://physics.stackexchange.com/questions/7446/travelling-faster-than-the-speed-of-light) the [accepted answer](https://physics.stackexchange.com/a/7457) says:

>For objects moving at low speeds, your intuition is correct: say the bus move at speed $v$ relative to earth, and you run at speed $u$ on the bus, then the combined speed is simply $u+v$.
>
>But, when objects start to move <b>fast</b>, this is not quite the way things work. The reason is that *time* measurements start depending on the observer as well, so the way you measure time is just a bit different from the way it is measured on the bus, or on earth. Taking this into account, your speed compared to the earth will be $\frac{u+v}{1+ uv/c^2}$. where $c$ is the speed of light. This formula is derived from special relativity.

What is "fast" in this answer? Is there a certain cutoff for when it stops being $u+v$ and becomes $\frac{u+v}{1+ uv/c^2}$?