If you are in an airline running a cyclic program the instrument rating does not expire for as long as you are in that program with that airline.
As soon as you leave the cyclic program you need to do an IPC to continue to use the rating.
Otherwise an IPC is valid for 12 months from last test - to the end of the calendar month.
The two year thing in the rules causes some confusion, but the way it works is if you fly two types, you can alternate the IPC test annually between types, but fly both types IFR for two years. i.e. you are still required to do an IPC annually.
Oh for the FAA system where the IR never expires as long as you retain approach and instrument flight time recency....though you still need a biennial flight review for private operations and within the airline system are obviously subject to regular proficiency checks.