Research demonstrates that kavalactones can degrade fairly rapidly when kava is not stored properly. According to Duve and Prased (1989) as much as 90% of certain kavalactones is gone after 39 months of storage in a closed (albeit not airtight) container.
Freezing is a good solution, but I would personally recommend vacuum-sealing. The key factors affecting kl loss seems to be moisture, so as long as your kava is vacuum sealed, it should be good and "fresh" for years.