You will need to wash your back support belt regularly, not only to keep it – and you – smelling sweet but to also help to extend its shelf-life. What you use your back support for will give you a good idea how often it needs to be washed. If you are wearing it for medical reasons and are not getting too active or sweaty in it, then every four or so days will be suffice but if you are wearing it for support during physical activity (DIY, working outdoors, at the gym or playing sport) then a wash after each use would be a wise idea.
When it comes to washing your back brace, follow the instructions carefully. Most material braces can be washed with a mild soap or laundry detergent and cold water. Always let the brace to air dry before wearing it again.