The ganache will go bad long before the nuts turn rancid. I think you can figure about 3 weeks for your truffles until they spoil, grow mold, etc. unless you freeze them. I am assuming you know your nuts need to be toasted prior to using them with your chocolates. Doing this accomplishes a few things but should also extend the time before they turn rancid. I've found old bags of untoasted nuts in the back of my cabinets after months and they seem fine. How quickly they turn rancid will depend on the oil content (plus storage conditions).