Probably you are asking about stability in media without cells and in sterile conditions. In the absence of serum more than with serum. But in any event I would not leave reagents in any medium for long. It is better to make aliquots of a concentrated solution (preferably not in medium -DMEM or else-), store them frozen and use an aliquot for each assay. Remember to vortex after thawing to homogenize the solution (freezing leads to an "ice" that is more concentrated with ML-7 or any other reagent at the bottom of the tube)