I have stored glop with a bit of thymol added as a biocide for 4 to 6 weeks in my refrigerator. I didn't notice any difference in the tissues I made with it. Generally though, I did as Andrew recommends and poured the tissue right away and stuck any tissue I wouldn't be using within a few weeks into the freezer.
If I remember correctly, I got thymol from Photographers Formulary and dissolved the crystals in alcohol making a 30% solution. A few drops added to your glop will keep it from turning into a science project in your refrigerator. Thymol is an ingredient in many brands of mouthwash. I have read posts by others saying that they have used mouthwash as a preservative - Never tried it myself.