I regularly purchase potting mix in the 2 cubic foot bags, and almost always more than I need for the current job at hand. Opened bags are dumped into a large trash barrel with a tight fitting lid and kept in a "bin" on my "potting bench". This is outside as I find it a much better place for such things. Unopened bags are stacked nearby. Freezing and thawing over the (long) winter takes care of any bugs and most other fungi problems. However, there are beneficial fungi that are found in certain types of soil and soil less mixes so I do not worry too much if any do show up. I do not bake or by other means sterilize the potting mix I use. and have not ever had any problems.
I use the same potting mix for all of my planting needs, from sowing seeds to transplanting to potting up blooming sized perennials, as well as all of my various types of house plants. The only thing I have even added has been perlite to increase the drainage and aeration for certain plants.
I've not had potting mix get "stale" per say, but I have had it get a bit too dried out. In which case I mix it 50/50 with a new bag. If I do not need to use it immediately, I will give the newly mixed media a good watering, a quick "stir" with the spading fork and then leave it alone for a few days with the lid off (unless there's rain in the forecast!)
If by some remote chance your bag of potting mix has gotten too wet and has soured, it can still be salvaged by opening the bag and dumping the contents onto the ground (preferably a paved surface) letting it dry out and then mixing in new potting mix. Cover this with a tarp and let it "cook" for a couple of weeks and it should be ready to use - in the garden or for potting up planters and barrels that will be used outside. I would also NOT USE that particular brand again.
Hemmaholic
PS
Potting soil rarely stays in a bag long enough here for it to "go stale".