We store in a normal 39F fridge/freezer with temp. control.
how long? well, right now i have over 40 mason jars either in 2nd, 3rd, or 4th generations, the oldest this year being 5 months old and when i stir plate them they are unstoppable. Use information as a baseline but not as the law, yeast are strong and can last for years. That being said, cells do die, but its the building up with a starter on a stirplate that brings it back. I dont advocate pitching jars or viles or smackpacks straight in.
Also, all that being said, We only go one year on washed yeast, then we get a whole new Wyeast supply. The yeast will do about 500 gallons, than they are retired.
Head Brewer of Kauai's Hindu Monastery