the idea of things wearing off, or us becoming resistant, comes from our use and familiarity with drugs, and indeed, they do wear off, and we do become tolerant/resistant. With this CD, especially if you use it every night, you are developing and strengthening 'association' and unlike medication, association goes on getting stronger.
The only way you can undermine association is to entertain negative ideas about it, such as 'it will wear off' (!), which is why I am replying, to say what I say to my clients in clinic, that once the ball is rolling you can start to take it for granted that it will just get stronger and stronger, and in fact, in taking it for granted we build expectation, and in sleep, expectation is a massive part of the process.
Very Best of Luck,