The dichlor and pucks, if kept in a sealed container should last almost forever. Check the labels for copper sulfate or zinc sulfate. The small amounts in them shouldn't hurt you, especially after a drain and refill. Otherwise just hang onto them for future use, should you need to bump your CYA after numerous backwash/rinse and vacuum to waste cycles. Understand that backwashing won't affect you much with the volume you have, but vacuum to waste could over time bring your CYA down a small amount.