How often does the metal filter plate in a stovetop pot actually need replacing?

There is no need to replace it on a schedule. It is a piece of metal, and metal with clean unblocked holes is doing its job.

The pack composition is about the gasket, not the filter. Gaskets are the wear item — they are rubber or silicone, they get compressed and heated twice a day, and they harden, shrink and stop sealing within roughly a year of regular use. A pot that hisses at the seam or takes ages to build pressure usually needs a gasket.

Selling a filter plate with every third gasket is a sensible retail decision rather than an engineering requirement: it covers the people whose plate genuinely has deteriorated, and it costs pennies to include.

Replace it when it stops working, and the failure modes are visible.

What to actually look for, since "when it stops working" needs a definition:

• Blocked holes that will not clear with brushing. Reduces flow and can cause pressure problems.
• Distortion. The plate should be flat. A bent or dished plate does not sit properly against the gasket and leaks grounds into the brew.
• Cracks, particularly radiating from the holes. These do appear eventually — I have had a plate crack after several years of daily use — and a cracked plate lets grounds through.
• Deep pitting or corrosion, which is more common on cheaper pots and on ones washed in a dishwasher.

If none of those apply, keep using it. Mine have generally outlasted several gaskets each.

Two maintenance points that matter more than replacement scheduling.

Do not put the pot in a dishwasher. Detergents attack aluminium, dull the finish, and accelerate corrosion of both the body and the plate. Rinse with hot water, and use a brush rather than anything abrasive.

Check the safety valve. This is the part of a stovetop pot that people genuinely neglect, and unlike the filter plate it matters. It should move freely. If it is crusted with mineral deposit and cannot release, pressure has nowhere to go — which is the one failure mode on these pots with real consequences.

A periodic check that the valve pin moves, and a descale if the water is hard, is worth far more attention than the filter plate ever needs.

Worth keeping a spare gasket and plate in the cupboard regardless of schedule.

They cost very little, they take no space, and the moment you need one is a morning when the pot is leaking and you have no coffee. That is the actual argument for the multi-pack, and it is a decent one.