IR skill test is around 60 minutes long
The applicant shall undertake the flight planning and shall ensure that all equipment and documentation for the execution of the flight are on board. The duration of the flight shall be at least 1 hour. That is required by Appendix 7(4) to Part FCL and recommended by GM1 FCL.1015(c)(5).
However an Flight Examiner will plan at least minimum of 3 hours for the skill test in total in order to comply with GM1 FCL.1015(b).
Additional time includes preflight briefing and preparation, conduct of the IR skill test, de-briefing, evaluation of the applicant and documentation.