sealing around back boxes

Regarding fitting back boxes and eventual sockets / switches etc. I understand that a gap is allowed between the metal back box and the socket / switch (the old "using long screws" etc). But what are the general rules about this gap ? The gap can be quite large after a previously untiled wall is tiled for example. Generally must be fireproof ? Is there are maximum gap allowed ? E.g. If you end up with a slight gap between the metal back box and plasterboard dry lining (say up to 15mm gap), is the general method to shove in some plaster / plasterboard adhesive etc into the gap so the plasterboard so the socket is roughly sealed from the gap behind the plasterboard ? Cheers, Simon.