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.
Pulled to the max:
View attachment 65793
Tight screw:
View attachment 65796
Loose screw:
View attachment 65794
Head extended/Fully Depressed:
View attachment 65795
Hope that helps.