Concrete – How long can I store a bag of Quikrete

Depends on where the natural water table is and how much pressure it can put on the pool (which is a function of the porosity of the earth immediately surrounding the pool). If you know the bottom is above the water table you can leave it empty until the ground freezes. Otherwise you're counting on the water table level and surrounding soil to not exceed the pressure from when it was first built or last empty without damage.

There is a countermeasure built into most pools: they have plugs you can unscrew or pop open when drained that provide a conduit for water to drain in and out of the ground. But again, those are only as good as the sub-surface porosity at relieving water pressure on the pool shell.

Concrete will never fully cure; it continually hardens forever, although for all practical purposes, it reaches a point where further hardening will be so slow as to unnoticeable.

Unless the mix has retarder (or some cement replacement material) in it, it will generally be hard enough to walk on in less than 24 hours.

Mixes are usually specified with a design strength - the compressive strength that the concrete will reach in a period of time, typically 28 days. As a general rule, at 7 days it will have reached around two thirds to three quarters of the design strength.

Don't worry about the water on the new concrete - it will help the concrete cure if it is kept damp after the initial set. The early stages of concrete curing is the hydration of the cement in the concrete.