Almost none — and that is the correct answer
A fancy mouse needs essentially no grooming from you. Mice are meticulous self-groomers: a healthy mouse washes its face with both forepaws, works through its coat several times a day, and grooms cage mates as part of normal social life. There is no schedule of brushing to keep, and attempting one usually stresses the mouse without improving the coat. What the mouse does need from you is observation — a short visual check a few times a week while it is active in the evening — and an enclosure clean enough that a self-grooming animal is not licking dirty bedding residue off its own fur.
What your coat checks are actually looking for
Watch for patches where fur has been trimmed short or removed entirely, especially around the face, shoulders and back of a cage mate. That pattern, called barbering, is one mouse over-grooming another and is a social or boredom signal, not a hygiene failure on your part. Look too for scabs, crusting around the ears, constant scratching, and a coat that looks spiky, greasy or thin all over. A mouse that stops grooming itself and sits hunched with a rough, staring coat is unwell in a general way — mice mask illness until they cannot, so a neglected coat in a normally fastidious animal deserves a prompt veterinary call rather than a wash. Weight loss, sneezing or a clicking sound when breathing alongside the poor coat raises the urgency further.
Why water and dust baths are both wrong here
Never bathe a fancy mouse. A mouse weighs very little, loses body heat rapidly when wet, and finds immersion frightening; water also strips the natural oils that keep that fine coat insulating properly. Dust or sand baths, which suit some other small pets, are not part of mouse care either — fine dust irritates a respiratory system that is already a mouse's weak point. If one spot is genuinely soiled, a barely damp fingertip or cloth corner on that spot is the most intervention a fancy mouse needs. A long-haired or satin variety may collect a little bedding in the coat; tease it out gently with fingers during normal handling rather than reaching for a brush built for a much larger animal.
The enclosure does most of the coat care
Because the mouse maintains its own coat, the condition of that coat reflects the cage more than any grooming session. Damp, soiled bedding mats fur and creates the ammonia build-up that damages both skin and lungs, so spot-clean wet corners daily and do a full clean on a regular rhythm, keeping back a little of the old nesting material so the mouse's home still smells like home. Wheels, tunnels and things to climb keep a mouse active and occupied, which reduces the stress-linked over-grooming that shows up as barbering. Handle your mouse briefly and gently most days: a mouse used to hands will tolerate the closer look you need when you spot something during a check.
When a check becomes a vet visit
Take a fancy mouse to a vet if scratching is relentless, if skin is broken or crusty, if a lump appears under the coat, or if the coat deteriorates alongside reduced activity or eating. Bring notes on when you first saw the change and whether cage mates show anything similar — shared signs point toward something contagious or environmental, while one barbered mouse points back to the group dynamic. Those dated observations are exactly what turns a vague worry into useful information at the appointment.
To keep the rhythm steady when several people share mouse duty, log the few-times-a-week coat checks in KinsLe using Care Teams, so whoever feeds the mice also knows to look the coats over and record anything odd.