Shelf Life of Powdered Hot Chocolate Mix

In my opinion, I would say a few months or so after the date.

My understanding of the 'sell by' date is that it is the last date the manufacturer will guarantee the product is of the same quality of the day it was packaged. So all it means is that after that date the quality will slowly degrade. Most packaged or canned items will be usable for quite some time after that. I have eaten canned goods that had a sell by date of two years ago with no ill effects. Obviously if the can 'bulges' or is rusty I would not trust it.