After dilution that three gallons from Costco should last a lifetime.
Unfortunately, no bottle of bleach lasts forever. It will deteriorate over time, slowly degrading to salt water. Keep it sealed and it will last longer. The shelf life is limited. Dilute it and the half-life drops drastically. The only adulteration in "pure" bleach is a buffering to raise the pH level to keep it from degrading too fast. They use Sodium Hydroxide to do that buffering. My primary doctor says they raise the pH to about 9-9.5.