Every six to eight weeks at most, and often less
The Siamese coat is one of the lowest-maintenance coats in the cat world: short, fine, glossy, and lying close to the body, with little undercoat to trap dirt or loose hair. A healthy indoor Siamese typically needs a bath no more often than every six to eight weeks, and plenty go a full season between washes without their coat suffering at all. These cats are also dedicated self-groomers, and their own daily routine does most of what a bath would do.
Wash for a reason you can name. A standing weekly bath, by contrast, strips the limited natural oil from a fine single coat, and the usual result is dry, flaky skin and a cat who objects more loudly each time, which anyone who lives with a Siamese knows is a considerable objection.
A short, warm, efficient bath suits this breed
Siamese are lean, muscular cats with little body fat over that fine coat, and they get cold quickly once wet. Fill the sink or tub with lukewarm water before you start, keep the room warm and draft-free, and plan a short bath rather than a long soak. There is nowhere for suds to hide in this coat, so rinsing is fast, but do not skip it, because a Siamese will groom residue off immediately afterward.
Many Siamese are intensely people-focused and will tolerate a bath better if their favorite person talks to them throughout, which this famously vocal breed tends to answer. Keep the conversation going, keep your hands gentle and predictable, and finish with a thorough towel dry in a warm spot. A quick brush or even a pass with a damp grooming glove before the bath lifts loose hair and makes the whole process shorter.
Between baths, a glove or soft brush is enough
Weekly grooming for a Siamese can be as simple as stroking her with a rubber grooming glove or a soft brush for a few minutes. That removes the modest amount of loose hair she sheds, spreads natural oils through the coat so the color points stay rich and glossy, and doubles as the social contact this breed actively seeks. During spring shedding, increase to twice a week rather than adding baths, because loose hair is the actual problem and a glove solves it directly.
This is also your chance to run your hands over the body and feel for anything new: a lean Siamese build makes lumps, sore spots, and weight change easy to notice early, which is a genuine advantage of a short-coated breed if you use the grooming time deliberately.
Coat changes in a Siamese worth mentioning to a vet
Because the normal Siamese coat is so sleek and predictable, changes stand out. A coat that becomes patchy, develops bald spots along the flanks, or grows dull despite good food and grooming deserves a veterinary look, as does over-grooming of the belly or legs, which in a high-strung, attention-loving breed can be linked to stress as well as skin irritation. Scratching focused on the ears or a sudden increase in head-shaking also points away from bathing and toward an examination.
Call your veterinarian if coat change comes with weight loss, increased thirst, or behavior change, since a breed that normally grooms meticulously stopping that routine is giving you information. A bath can make her look better for a day; it cannot tell you why she changed.
With baths this occasional, the record matters more than the routine, and KinsLe Care Teams lets whoever bathed your Siamese log the date and any coat notes, so the next person who wonders whether she is due checks the profile instead of guessing.