You can extend shelf life by adding glucose, corn syrup, invert sugar or alcohol to the recipe. also be careful about not leaving any air bubbles inside the truffle, or it'll be a certain spot for mold. I boil my cream and let it rest until it reaches 30C, then I mix it with the tempered chocolate.
Haha I'm not sure actually, they never last that long, but I've opened them within 2 weeks and they're still perfect. I've read that using glucose, boiling the cream and using tempered chocolate makes your chocolates last very long (about a month). But it also depends on your recipe.
You can also freeze your ganaches, and finished truffles as well :) so you can work the heavy seasons in advance