An IR is valid for 12 months, so you have to do a licence proficiency check (LPC) once a year.
An operator will be subject to whatever regulations the competent authority decides, in most cases that means an operator proficiency check (OPC) in between.
The result is usually a SIM every 6 months, and there's hardly any difference between the two (eg an LPC requires a manually flown go around and single engine approach, an OPC can be flown with the automatics) so it's quite common for a crew to have a SIM session with one doing an OPC and one doing an LPC.