NutriGrubs are one of the longest-lasting, most "shelf stable" live feeder insects. While it's not a guarantee, properly stored black soldier fly larvae can survive for up to 9 months! 2 to 3 months is a more reliable timeline to expect them to survive at an ideal temperature.
Cold Storage
The trick to achieving a lengthy shelf life is storing your NutriGrubs at the proper temperature. You can put them in the door of your fridge, but you need to ensure the temperature doesn't dip below 50°F, which is difficult in most household refrigerators. A more reliable option is a wine cooler that allows you to set the temperature to 50-55°F manually. Here are some other creative options:
Mudroom
Enclosed porch
Drafty windowsill
Basement or attic
If you can't store them in an area that reliably stays above 50°F, keep them in the coolest room of your house. The cooler it is, the longer it will take them to pupate.
There are reports of NutriGrubs temporarily surviving temperatures as low as 24°F, but there's no telling how long they'll withstand those conditions. Refrigerate them at your own risk and discretion.