It is safe to clean your walls two weeks after the final top coat is applied. Most interior paints in the market today are all washable.
It is recommended that you dust and or wash your walls once a month. A soft headed vacuum can often be enough.
Test a patch in an inconspicuous spot first. If paint chalks onto your sponge, do not wash that paint. Using a soft sponge or microfibre cloth and a warm, mild soap solution (see below for some homemade recipes), lightly rub any area where a wear mark or stain has appeared. Do not run excessively (especially on a stubborn stain) as this may damage your paint work. A tip for stubborn stains is to use a solution of baking soda and water and gently rub the stain.
Always rub in a circular motion to avoid damaging your paint and work in small sections. Once you have finished a section, dry that area with a clean soft cloth.
Take particular notice around light switches and areas behind the couch where someones hair may leave a grease or oil mark.