Just remember that seed in a seed bank is perishable. While it does last easily five years, and probably ten years, under ideal storage conditions, I think it would be reasonable to aim for a renewal grow-out at least every five years. That would be 20% of the varieties renewed every year by reliable growers. Although the numbers aren't that large, it does pose an administrative challenge, just to keep track of who, what, when and where.
Bob
Bob