Longevity of concentrated STAB

I have a bottle of concentrated formaldehyde based Tetenal STAB (stabilizer, 1:100 mixture ratio for creating stabilizer solutiion for E6/C41) that I have been using over the last year for slides. There is no printed expiration on it, but I believe it may have been made in 2010. Does anyone (PE?) know if formaldehyde based stabilizer lasts for a very long time? Or is it prone to oxidize or break down over time once opened?
Thanks in advance.

Formaldehyde solutions can become cloudy or throw down a white powdery precipitate. This is caused by polymerization of the formaldehyde to paraformaldehyde. However this can be prevented by the addition of a small amount of methanol to the solution. I would assume that Tetenal does this but you can check the MSDS for the stabilizer. As long as the concentrate is clear it should still be OK.

Tetenal states in their instructions that opened concentrate bottles last for 24 weeks, the same as used working solution, yet I have never seen a precipitate forming in STAB well over a year old. This makes me wonder whether there is another mechanism at work which decays the STAB. Would Formaldehyde eventually diffuse through the bottles/lids and render the liquid ineffective? What about the biocide (1,2-Benzisothiazolin-3-one), is it prone to aerial oxidation?