DrFirst recommends setting aside 10-15 minutes of dedicated time and having the information you need handy. See article: "What information is needed to register for iPrescribe?".
You may pause and return to registration at any point if you cannot do this in one sitting. It may take longer if there are extra identity proofing checks while working through the process, each person's experience may differ. However, on average 10-15 minutes can be expected.