Expiration date for Potassium phosphate buffer? - (Mar/14/2008 )

I have potassium phosphate buffers that I use for solid phase extraction and HPLC mobile phase. What is an appropriate shelf life/ expiration date for these buffers if they are stored at 4 degrees C?

25 mM KH2PO4 pH 3 for solid phase extraction
62.5% 0.02 mM KH2PO4 pH 3.5/ 37.5% acetonitrile for HPLC mobile phase.

And most importantly, could you please cite any references for your answers? I've searched for hours and found no answers so far.
Thanks very much!

i tend to throw stuff out like a phosphate buffer, after a year...

if i'm doing something important, i will make it up fresh (rather than risk it with an old, potentially contaminated buffer).

i was told this by a senior RO, so there aren't any papers to back it up. still, it is better to make a small amount that get's used quickly, than to make large batches, that sit and can be contaminated.

V