The shelf life of chalk is 33 years, 7 month, 14 days, 11 hours, 5 minutes, and 44 seconds.
Seriously, I'm with everyone else ... forever, or at least beyond a human life span if kept dry.
Even if allowed to get damp I think it would still be good after a good drting out.
In a humid time of year or climate a dessicant such as silica would help in storage I imagine.
If you really want to be uber cautious Marv, nearly every bottle of vitamins or over the counter pill will have a small anti dessicant pack inside. Tos a couple inside your baggie of chalk and call it a day.
I use them in camera bags and my cue case also.
LWW