Shelf life of ifixit battery kit?

I was getting battery error messages any time the power adapter wasn't plugged in. State of charge was stuck at 1% or went from 100% to 1% soon after the adapter was unplugged. The battery preferences tab said that it needed service. So I ordered up the iFixit replacement kit.

Step one: drain the old battery. Well, darn if it didn't run for nine hours of writing, browsing and reading on "1% charge." Seems as if it's more of a calibration problem than an end-of-life problem. Though the preferences tab still says it needs to be serviced. There appears to be quite a bit of life left in the old battery after all.

So my current dilemma is whether to replace the battery anyway (some rainy day when I "get around to it"), hold the Ifixit kit in reserve (for months? years?) or return it. How quickly will the new battery degrade, just sitting in its original packaging?