Bacterial stocks should be made at a final concentration of 15-20% glycerol and stored at -80C. Viability can decrease over time, but these stocks will last many years (decades at least) if stored properly.

From past 4 years i have been working with glycerol stocks and up to my experience 15% was the best to use as you increase the glycerol percentage that will be more toxic to cells. Also with the increase in Glycerol concentration the cells should be kept at -80 only. If you take the stocks out it should be rapidly thawed and added into fresh media within 1-2 mins if it takes more time defenitely you will loose Viability.