How often should I replace a steam mop or microfibre pad?

When it stops holding water and starts pushing dirt around — typically after a few hundred washes, and much sooner if it has met fabric conditioner.

Microfibre works because the fibers are split, which gives an enormous surface area and a slight electrostatic grab. Fabric conditioner coats those fibers and destroys both properties permanently in one wash.

The other killer is heat. High-temperature tumble drying melts the fiber tips, and the pad becomes a smooth cloth that smears rather than lifts.

Wash them hot but without conditioner, dry them on low or in air, and keep enough pads that you never restart a job with a dirty one. Two per floor is a reasonable working number.