If I recall correctly from the Charlie Talley show, he said it is basically an acidified soap. Once the pH climbed above 3.5, it was ineffective. You could "rejuvenate" it by adding acid to lower the pH.
So I guess it would be slightly acidic, soapy water.
The 'soap' (and it is technically considered a soap) is the extremely popular detergent dodecyl benzene sulfonic acid. Given that it works at cell membrane lysis by having its non polar tail dissolve in the membrane lipids while its negatively charged head is attracted to water molecules one migth conclude that it would be more effective at higher pH as it is more charged at higher pH and this may be true but then remember that the killing is actually done by the protons released from the phosphoric acid and therfore the system is not as effective as pH rises. Exposure to air should not, by itself, be detrimental. Oxygen (translation - acid generator) isn't going oxidize the phosphoric acid and I doubt it would hurt the DDBSA either but I really don't know that.
What will do in the phosphoric acid is alkali. When phosporic acid is mixed with alkali the following chain of reactions takes place
H3PO4 + 3(OH-) ---> H2PO4- + H2O + 2(OH-) ---> HPO4-- + 2H2O + OH- ---> PO4--- + 3H2O
with the pH rising as you go across the page. If the water is hard (contains lots of calcium) as soon as the pH gets high enough calcium phosphate (hydroxyl apatite) will precipitate as well and this is responsible, I believe, for the reports one sees that the solution turns cloudy. Precipitaton removes phosphate from the solution.
So I think you will get more mileage out of it if you dilute with RO or DI water. I guess you could 'rejuvenate' old solution with phoshphoric acid but unless the stuff is really expensive I'd just throw it out at that point.