White labs has a best by date on it. I believe that Wyeast does also. But, even if it is beyond its 'expiration' it can still be used. In either case you should make a starter and adjust the starter size depending on how old it is to pitch the proper amount of yeast. http://www.mrmalty.com/calc/calc.html
I wash yeast as well. The oldest that I've used is 4 months, but this weekend I'll be using a strain that was originally purchased 9 months ago. I keep making starters off of the original, unused in a brew strain. I've used it probably at least a dozen times.
Originally Posted by the_bird
Well, if you *love* it.... again, note that my A.S.S. has five pounds.
I just started washing. So far, so good, and it really saves $$ not having to pay $8 per batch for liquid yeast.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10