RE Enzyme Aliquot Dates - Just from curiousity... (Mar/20/2008 )

How long do people keep their restriction enzymes around past the aliquot date? NEB enzymes I believe come with the month and year they were tubed (is that even a verb? ) on the label. Digging in the -20, I found some from 2000... 2001...

We have some from '97 that still work fine.