Short answer: once a week, twice if three or more people use it. Guest bathrooms that go unused still want a fortnightly flush and wipe, because standing water leaves a ring and the trap dries out.
The parts people miss
Cleaning the bowl is the part everyone does. These are the parts that decide whether the bathroom actually smells clean:
- Under the rim. Where staining and bacteria concentrate. Needs an angled brush, not a straight one.
- Seat hinges. Most lift off or unclip. Almost nobody does this, and it is where the worst of it collects.
- The base and the floor around it. The single most common source of persistent bathroom smell.
- Behind the cistern, and the wall behind the toilet.
- The flush button or handle, one of the highest-touch surfaces in the house and rarely disinfected.
If a bathroom smells despite a spotless bowl, it is the base and the floor seal. Every time.
A workable routine
Weekly: cleaner into the bowl, leave it while you do everything else, brush under the rim, then wipe seat, lid, hinges, cistern, flush button and base with disinfectant. Mop the floor around it last.
Monthly: lift the seat off entirely if it detaches, clean the fittings, and clean the wall and skirting behind.
Quarterly: treat limescale in the bowl if you are in a hard-water area. Waikato water is relatively soft, so this is less of an issue here than in parts of Canterbury.
Two things worth knowing
Leave the cleaner to dwell. Toilet cleaner needs several minutes of contact time to do anything. Squirting and immediately brushing is mostly theatre. Apply it first, do the rest of the bathroom, come back to it.
Close the lid before flushing. Flushing aerosolises the contents across the room, including onto toothbrushes. This is well established and costs nothing to act on.
Never mix bleach and acid-based cleaners, including some limescale removers and anything containing vinegar. The combination releases chlorine gas. Use one product, rinse thoroughly, then use the other if you must.
Keeping a brush that is not itself the problem
Rinse the brush in the flush water and let it drain fully before it goes back in the holder, resting a wet brush in a closed holder makes it a worse contaminant than the toilet. Replace it every six months; they cost a few dollars.
For what the rest of the bathroom needs and when, see how often should you clean your bathroom.