Well, it doesn't exactly extend per se, but the 'inner' sleeve depresses about 3/4", though the Doctor rarely presses it that far. It's important to note that the range is also dependent on the the set screw in the tapered head, as at times the sleeve goes far enough to touch the emitter halo, and at others it leaves a couple grooves exposed on the central post.
In "Genesis of the Daleks", the emitter itself did extend about a 1/2", but only by pulling on it.