That isnt easly answered. Did the nursery provide a microbe analysis? Certified compost is available and is expensive (via the internet of course). The science exists but there seems a leap of faith when business is involved. You will have to rely on smell. How does it compare to your home made compost? can always mix the two. Lets hope you only need a few to survive then they wake up & go crazy reproducing in the tea.

Fortunately, bacteria spore up when conditions aren't quite right (low food levels, too little water, too hot, too cold, etc.) I'm not sure about fungi, but I can't imagine you won't have plenty of spores in there waiting for the right conditions.

There's a limit on time. However, it's been estimated that the bacteria on our discarded booster rockets coasting through space have a 10,000 year lifespan. Or more. As long as they're on the inside and protected from cosmic rays, anyway.

I think I got that off Bad Astronomy, but I could be wrong.

Roughly, I'd say use the bag in one growing season. Whatever's left you can scatter lightly over the lawn and/or gardens in September.