In article , "Dave Plowman (News)"
says...
In article ,
Rob Morley wrote:
Eh? Pollyfilla that thick will take for ever to dry - and shrink
anyway.

How thick?

Thick enough to use to hold the box in place properly. Or anything other
than a thin coat - it dries in air rather than a chemical reaction like
mortar.

It doesn't need to be very thick (unless you've made the hole much too
big) and the surounding masonry draws the moisture away quite
effectively.