Storing your UTV in an environment where it is exposed to dirt, dust, debris and moisture will shorten the life of the belt. Most UTV belts are made of a natural rubber compound that will absorb small particles from its environment into the pores of the compound. This negatively impacts the coefficient of friction resulting in an increase in heat and earlier failure of the belt. Storing your belt where it is exposed to UV rays or where the belt is in contact with other objects can create cracks or weak spots in the belt resulting in early failure as well.
Check out KWT Filter's article on Maintenance Tips here, which stresses the importance of inspecting and taking care of your UTV's drive belt.