That’s not quite correct. Each MOU region has it’s own criteria, in some cases these can be found on the website.For example Paris MOU is 24 months but can be shorted depending upon specific criteria .
Here is from Paris MOU site:
The targeting of ships is based on a “Ship Risk Profile” (SRP). The SRP Calculator can be used to evaluate if a ship will be considered as High Risk Ship (HRS), Standard Risk Ship (SRS) or Low Risk Ships (LRS). An element which helps to categorise a ship is the inclusion of the performance of the ISM Company. Companies are, as the flag and recognised organisation, ranked. This ranking is done in four distinct grades: “above average”, “average”, “low” and “very low”. The Company Performance (CP) Calculator takes historical events such as detentions, deficiencies and good inspections of the complete fleet of that company into account. THETIS automatically re-calculates the SRP on a daily basis, taking the latest inspection information into account.
LRS are rewarded and not subjected to an inspection within the 24 months following the last inspection, with a maximum inspection-free timespan of 36 months.
SRS are subjected to an inspection interval of between 10 and 12 months.
HRS face the more stringent inspection interval which is every 6 months. The inspection on a HRS will be an in-depth, so-called “expanded inspection”. These expanded inspections include risk items which will be inspected every time.
All that goes out the window if the ship enters a port in a different MOU area. Then the ship is subject to the criteria of that MOU area regardless of what PSC inspection it may have had in a previous port in a different MOU area.