How long can yeast be stored?

Theoretically yeast can be stored indefinitely. Yeast goes into a state of dormancy once all of its food is gone. However the yeast might die if not stored properly. If you plan on keeping a yeast culture for more than six weeks, it's a good idea to make a starter prior to pitching. You do this to ensure viability of the culture, and also to help "wake up" the yeast.

Here's a link to a BYO Magazine article about yeast storage and reusing as per request.

Wyeast liquid yeast has a 6 months warranty shelf life. http://www.wyeastlab.com/rw_advantage.cfm

Danstar dry yeast comes with a 2 years guaranteed shelf life, if stored properly. http://www.danstaryeast.com/articles/dry-yeast-storage

Fermentis claims their dry yeast is viable for 2 years from the date of manufacture, as long as the yeast is stored properly. http://www.fermentis.com/brewing/homebrewing/

It seems 2 years for dry yeast is the standard shelf life and 6 months for liquid yeast.

I also read on a manufacturer's website (can't remember which one) that a percentage of the yeast decays after X number of months. So after 2 years, your dry yeast pack might have 50% of the yeast active. It doesn't drop to 0% at once, it's a gradual process.

Short answer:

Your liquid yeast should have an expiration date. Most dry yeasts will as well. This means that the yeast within is only going to be reproducing and active for that long. Typically 3 months after shipped.

Long answer:

When yeast run out of food or are under other forms of stress, they will enter a sporulation phase. During this process, the yeast will transfer vital cell parts and DNA in to a spore. Spores can live very long periods of time. I'm not sure how long yeast spore live but other Fungi spores have been known to live for upwards of 250 million years.

You should be able to make a yeast starter and bring yeast spores back to an active state within a couple days if feed and stored at ideal reproductive temperatures regardless of how long you've had them.