My medical grade Glutaldehyde, Metricide, has printed shelf life of 2 years on the bottle. But once activated with the stabilizer, it has to be used up in 14 to 28 days as labeled in Metricide 14 and Metricide 28, respectively. I’m not certain the purpose of the stabilizer which is sodium nitrite, so don’t ever add it in aquarium use as it is toxic to fish.
I have big tanks and use up one gal glut in a year. I dose at 2 ppm (initial high dosage per Seachem) after weekly water change as a preventative measure against algae. I figure it is better off use up the bottle once opened as the content has finite shelf life. I have no visible algae on the plants, but it may be helpful to prevent algae on the glass lid and hardscape surfaces.