It is always better tu use fresh solutions or reagents if we have any doubt about the outcome

Marc, thanks for your response. I know it's better to always use fresh solutions. However, we do this DNA isolation almost weekly, so it seems like a waste of material/money to throw remainders away every time.

You can store it but SDS should be avoided. I usually make buffer solution with out SDS and with Prot K. Add the SDS at time of making tissue lysate. Mine is almost a year old kept in 4C still works fine.