How long
Yes of course if the candidate is very much on the ball then it can be completed very quickly. I suspect Drauk that your either a very talented and exceptional learner or perhaps you already had considerable experience before taking your RT test.
10 minutes of planning and the test completed in 20 minutes is hardly a pace that an average student yet to graduate will achieve. Whatever it is not for the Examiner to push the pace. The purpose of the event is an attempt to eradicate the poor RT of which you speak. Like any test it is best when the candidate leaves also having learned from the experience. With any written exam that they will undertake they will only have had to demonstrate 75% of the knowledge questioned. A practical test will also require a degree of error by the student and may reveal a weakness in knowledge which must be assessed and should then be debriefed. The Examiner must then decide whether it is a simple matter and resolved by debriefing or whether further training or study is required. 10 + 20 and out the door is hardly providing the candidate with the service that they should expect and for which they pay.
An oft used quideline for planning a trip is that the student will take twice as long as the flight duration being planned will be. Quite typical in my experience. It will not be unreasonable for an RT candidate to take 40-60 minutes to plan the RT test. I also recommend that they plan a script for the RT situation is unusual being sat in a room using a box of tricks. It is very easy to become lost in time.