A LONG LONG time. If it's sealed in a ziplock, and sitting in a dark pantry it'll keep for years. I drank kava from 2015 earlier this year because I had failed to notice my kava stocks and ran out. Just make sure it doesn't smell moldy or "off" when you open the bag for the first time in a while.
I would guess 10 - 15 years with proper storage. If kept dry and intact, pepper products are amazing, even though the active ingredients involve lipids. Dry peppercorns, for example, if whole and stored dry, will keep for 100 years.