I have saved yeast and used as many as five times...I haven't had a problem yet. I've never ventured beyond this, as I get progessively worried about contamination. However, I suppose that another use or two wouldn't hurt.

I either save in the fermenter if using within the next few days, or if I plan on not using for awhile, I put it into a sterile mason jar with the lid slightly loosened to allow gas escape.
-Brian