The official expiration date for N is 2 years. If stored properly, it has been known to last for 20 years. The N loses about 1/2 percent of it's potency per year. The liquid should be clear. If it turns cloudy and you see solid specks floating in the liquid (precipitation), the N is no good, and you shouldn't use it.
It's fine to store it in your wardrobe. The N should be stored at room tempature. The label on a bottle of N says
Store at 20°-25°C (68°-77°F); excursions permitted 15°-30°C (59°-86°F).
Just make sure that you don't open the bottles until you are ready to use it. Store the bottles in a dark airtight container. It's important to keep the N away from light and moisture as this causes the N to degrade faster. You want to store the N at a constant temperature and avoid extreme fluctuations in temperature.
2 bottles contain 12g of N. Technically 2 bottles is overkill. It is more than enough to get the job done. The lethal dose of N. is only 2 g. One bottle contains 6.5g of N, and is enough to kill someone. Some people get to bottles to speed up the process.