What is the shelf life of HTV (Heat Transfer Vinyl or iron-on)?

HTV such as ThermoFlex Plus or ThermoFlex Turbo doesn’t really have a shelf life like adhesive vinyl since the adhesive is heat activated. That said, it needs to be stored properly so if it’s exposed to extreme conditions of temperature/humidity it can cause problems, usually delamination from the carrier sheet. With some like glitter HTV the delamination is more likely to occur over time even when stored properly but typically if stored properly it would usually be good for at least a few years in most cases. Ideal storage conditions for vinyl of all types is recommended at 70 degrees Fahrenheit and 50% humidity. The further you get away from that the shorter the shelf life. Those storage conditions would typically exclude transportation and short-term exposure as long as it’s not extreme. If you ordered vinyl from a company and your package is outside on your porch in freezing weather or in 104 degree heat in direct sun for 2-3 days, that may affect the product, though.