I live by the "Five years or 300 days" rule, but generally stick with whichever comes second. At 5 years it may be 600 days for me.
Don't just make sure it floats you high. Compare it by trying your friend's brand new PFD right before/after. Also, visually inspect straps and buckles.
Finally, when it no longer works well for, please throw it away and don't try to sell it to another boater!