While you can technically save drywall compound for years in the right conditions, the reality is that most of the time the shelf life on drywall mud is far shorter. Manufacturers have a specific date of recommended shelf life on their brands, which is basically the point beyond which any warranties are null and void. If you want to ensure your work is warrantied, you have to use drywall compound within its official lifespan.
If you are a die-hard home-improvement veteran who keeps things around the garage for a rainy day, drywall compound can take up shelf space for years as long as the proper conditions are met. However, the problem is that drywall dries up over time, and after the recommended time frame from the manufacturer the compound begins to lose strength and potency. You use drywall compound at your own risk beyond the expiration date.
Average Shelf Life
Different manufacturers have different shelf life recommendations for their products. Most wet compounds are rated for around nine months to a year so long as the compound is kept in the right conditions. Dry compounds have a similar shelf life of around a year so long as the proper storage methods are used.
Perhaps the most important aspect of storing drywall compound is ensuring that the compound's container is completely airtight. For pre-mixed compounds that are in a bucket, this means ensuring that the lid is locked in place before the mix is stored. For dry mixes, you need to store the bags in some form of waterproof and airtight container where it is protected from the elements.
While it doesn't affect the dry compounds, any mixed compounds you have saved in a container cannot be allowed to freeze during the winter, as this will dry the mud out. Once drywall mud is allowed to dry, the compounds within the container slowly separate as the moisture is kept from softening things, eventually hardening into a solid form in the bucket even after it thaws. One freeze is sufficient to void the warranty.