If you adhere to any third party scheme or customer requirements I'd have a scan through to check no figure is mentioned, otherwise there is no strict height per se as far as I'm aware.
Looking at BRC for example, from the interpretation of clause 4.11.5:
Equipment must be clearly identified at all times (by colour-coding and/or labelling). This is necessary to ensure that different types of equipment are clearly distinguishable (e.g. floor-cleaning equipment must be distinguishable from items designated for cleaning production equipment). They must also be stored hygienically and in a manner that prevents contamination (e.g. not stored in contact with the floor).
Having a quick google, I notice an extract from "Hygiene Design of Food Factories" recommends the following:
Storage of cleaning equipment should be considered, and if the cleaning equipment is often used wet, it should not be stored in contact with the floor. If wall mounted, the head of the item, e.g. floor brush, should be approx 0.5 m from the floor with the handle above. High risk/high care cleaning equipment should be stored dry or in disinfectant.
I imagine 6 inches would be deemed high enough from the ground to allow the equipment to dry and reduce contamination, but also low enough to reduce contaminating staff PPE shirts or food racks accidentally knocking against it which is worth considering.