pH down should have an indefinite life span. pH up's life span is based on how often you open the bottle and how long you leave it open. Each time you open the pH up bottle air enters the bottle so CO2 is introduced. This neutralizes a little bit of the hydroxide in the pH up lowerings its overall strength. The only thing that could happen to the pH down is an increase in strength as water evaporates the solution would become stronger. fatman 7574