some people say it starts losing effectiveness in like 3, or 5, or 10 years.. but other who are that many years in say that they have not experienced significant loss and that its working.. it's all on who you ask. I'm sure eventually your body will become immune to it but by that time we'll likely have better treatments.
As you alreayd know, the real issue is the receptor, not DHT per se. Propecia may still be very successful in reducing DHT levels. But the point is rendered moot if your hair has an abundance of the androgen receptor. Again, it's the recptor, not DHT, that's the cause.