What would be a good interval to re-qualify the equipment? Would it be necessary to perform IOPQ again?
It’s up to you to define and to define the necessary activities. You should have a Master Validation Plan that provides the guidance.
What you do cannot be arbitrary, though. You can’t just say “we’ll do re-qualification every 2 years by just doing IQ.” The decision / approach has to be risk-based and justified.
Frankly, there’s no requirement just for re-qualification. The system / equipment needs to always be in a validated state. You need to define what could occur that would take the equipment out of a validated state.
Changes (direct and indirect - like an OS upgrade if the equipment has software on a processor that runs an OS), maintenance, cleaning, repair, etc. all are events that need to be considered. Then you define how you confirm the system is in a validated state after such events. I think most typical is a subset of OQ and maybe a reduced PQ. But again, this has to be justified (documented).