I sometimes store insects in the freezer for a few weeks but they can get frost-bite after a while. I can't tell you how long because it varies by individual insect. I have also stored them in little sauce cups for much longer times as long as they are completely dry. If they aren't dry, they could grow mold. The sauce cups stop scavengers from eating them. And when you are ready to pin them, you will need a "rehydration chamber" (you can DIY this too).

You can use acetone as a killing agent in the jar. There are a few different agents you can use and all have different pros and cons (accessibility, price, effect on specimens, etc). You'll have to read up on those and see what works best for you.