Some people keep them in the refrigerator but I think that as long as it's not in direct sunlight just by being in alcohol above 20% it will already last for a long while (at least an year). If you are storing in a dark glass bottle or container at constant temperatures it should last a while. Just give it a shake before you use if you leave it for too long, and obviously smell, taste and see if it looks okay, but it shouldn't be a big problem.

I see about the lucid dream. Interesting experience though. I found that Mugwort made me sleep longer and be able to get back into dreams easier as well.