I would stock up!!!
even if it does expire you could refresh it with I don't know maybe compost or fertilize.
It probably has a time released fert. like osmo. maybe thats what expires.
I don't know about no name miracle grow. I don't like using that stuff, but a lot of people do.
I heard it's "plant crack"
the blue stuff right? unnatural color too.
not a fan.
Laura

If that potting soil was kept dry it should be good for many years, depending on what was in the mixture.
I do not and have not used any synthetic "fertilizer" so do not know whether one is as good as another. If you get your soil built up into a good, healthy soil those products are not necessary.

I agree that moisture is the key. If you can keep the soil dry, it will last indefinitely.

You don't 'build-up' container soils. Hopefully, you start with a soil with good structure that has built-in longevity, because it's certain that from the moment a planting is established, the soil is on its way to structural collapse. If you want your plants to grow at near their genetic potential, fertilizing, in container culture, is requisite for all but the odd plant here & there.

On a scale of 1-10, with 1 being growing in the garden or beds, and 10 being hydroponics, container culture would probably rate a 7 or 8. Container culture is MUCH closer to hydroponics than it is to growing in the earth. We absolutely KNOW that hydroponics requires the continuous or very regular addition of nutrients to the medium, so it is going to be difficult to support the idea that you can simply mix a pile of ingredients together in spring, plant in it, and then forget about the plants nutritional needs.

No, you cannot depend on 'building-up' a container soil for the ride unless you're willing to accept plants that are growing at considerably less than their potential vigor. Somehow, you must fertilize.

Even if you fertilize and supply nutrients via nothing but organic sources, it's more likely that you're accelerating the collapse of the soil than building it up. Much of what works in the garden is better LEFT in the garden. Often, trying to adapt even practices that prove stellar in the garden leads to abject failure in container culture.

Al