I usually buy it in quantity because its cheaper per pound that way. I've never had an issue keeping it over the winter for use the next season.

Your cya may or may not stay level over the winter. There are bacteria that will eat it. I've had some winters where cya didn't drop much if at all, and others where cya was completely gone by spring.

If cya does get eaten up over the winter, be aware it will take alot of chlorine to burn up the byproducts when you open in the spring.