Carbon that has been opened looses its cleaning abilities a lil as it starts to absorb stuff in the air. "Reactivating" it is more like giving it a wash, as real activation reqires huge pressure (50atm or something) and 900 C temperatures.
I home distill alcohol, and you can get a bit of extra use out of carbon that has absorbed some impurities by giving it a wash in bioling water or in the microwave. Look it up before you chuck it in the microwave.
Cleaning will never get it back to its original state - basicially by using heat you are trying to re-vapourize the molecules trapped in the carbon, and many will, but not all will go. So it gets worse over time.
I would say that with an opened bag you will be fine!!! especially since its only gases absorbed- very likely to re-vapourize, if you wash it.
It's surprising how fast an opened bag of carbon looses its cleaning ability.
It doesnt have a "shelf life" if in a sealed container.
If you are that worried about it dont use it, for peace of minds sake. Personally I would use it after a good washing, - I drink the products of re-used carbon and I'm fine. Well, slightly inebriated!!!!