Wifi repeater distance

The answer is not that simple. It depends on many things like how powerful the transmitter is and if there are any obstructions in the path of the signal. In an completely open area the signal could possibly cover 30m at best...
 
As ngrego said, there are many factors, like what materials the building is made of and how many walls are in between the repeater and the computers using it. One factor is the type of repeater you're using - there are some high-powered repeaters meant for larger areas; there are also some really powerful ones like the Amped SR600EX that are meant to be aimed in a particular direction (instead of extending into an area that forms a circle around the repeater).

There are a few things to take into account when setting up a repeater in your house or office that can maximize its effectiveness. Try taking a look at this article for some insight: wirelessrepeaterhq.com/wireless-repeater-positioning/.