I am currently stepping up some washed pacman yeast from over a year old. It had been washed three times before storage (in February of 2011). It took about 24 hours to show signs of life but its going strong now.
I've saved washed yeast for several months. Make sure to make a starter with it if you let it sit for more than a couple weeks. Mr. Malty is a great resource for just repitching a slurry or making a starter. You can input when you harvested the yeast and it will estimate viability based on this.