Hello Sean2,
Welcome to the DigiKey TechForum! As Rick stated previously, the service lifespan and the shelf life are two separate things as the impact on the component is different due to differing environments.
I was not able to find a service life listed on the documentation for the GP1500, however I will say that generally when a thermal pad is applied it is usually done so with the intent that it will last the lifetime of the component it is applied to. This comes with a few different caveats, of course. Not applying it correctly, straying outside of the listed maximums, and using it for things other than it’s intended purpose are all factors that can reduce its lifespan.
These parts are made for and tested in their service environments as well, and so keeping them in an environment they’re not specifically made for (aka the shelf) tends to have an adverse effect on its overall lifespan as well. The manufacturer includes a shelf life so that if the product is used before 6 months of sitting on the shelf, it can be assumed with high confidence that any issues that arise for using the product are not due to it sitting on a shelf. Then then allows someone to check other avenues for root cause analysis.
For more information regarding Shelf Life and what it means for components, check out our posts about it HERE and HERE.
Regards,
Klint