See post no. 10 here:
Stock solutions mixed in glycol should last at leat 2-3 years.
But Xtol concentrate contains some water of crystallisation from the metaborate.
I am running a 1 year test on early Xtol concentrate D-316 due to complete Jan 2013 which should show if this water of crystallisation has an adverse effect on shelf life.Even if it does it should be possible to make a recommendation to heat the concentrate when it is made, to drive off this water.