Generally, Brilliant Switches should be within three rooms of each other or a Brilliant Control.
Brilliant Dimmer Switches and Brilliant Plugs need to be able to talk to a Brilliant Control. They can do this by automatically "hopping" from one to another until they reach a Control. This all happens automatically, with no configuration required, and it means that Plugs and Switches can function well even where there is a bad Wi-Fi signal. This can happen over any distance, as long as each Switch or Plug is close enough to another one to communicate, and ultimately reach a Control. How close this is depends on the construction of your home, but generally, each hop from one device to another should be within about three rooms apart. As long as there is a way for each Switch or Plug to hop from one device to another and reach a Control, the whole network will work.
If you find that you need a signal boost, try putting a Brilliant Plug in between devices so they have shorter hops when they talk to each other.