I use protein sparingly, and it seems to last forever, at least in a tub. Probably depends what sort of environment it's been kept in. I agree with Ryano's caution, but your nose should be a reasonable indicator. You could always try an itsy-bit, and, if you survive the experience, try a bit more.
Lifetime best: 500/363/573 @ 220 belt only
"The proper study of mankind is books" - Aldous Huxley