This is an excellent question.
Over the last five years we have endured not less that two interruptions in the supply of enzymes each year (not just with the Kelley enzymes but the 1200 pancreatin) and even (the enzyme community is small so I hear from other suppliers) the enzymes that Dr Gonzalez recommends has seen interruptions.
So yes I recommend you stockpile what you can afford.
The longest we have endured has been six to eight weeks.
If you do stockpile enzymes be sure to rotate the stock using the older stock first and generally order once or twice a month to keep a steady flow of enzymes on hand.
The "official" shelf life is (my understanding) two years.
I do not recommend you stock more than you will use in three months. This should give you a large safety buffer without any degradation of the potency of the product.