Don't listen to jeffsss he is quoting one peiece of info that in no way represents all cases. Further more he has been trolling the forum quite heavily due to his shedding, so in my mind he is losing his credibility.
finasteride does seem to loose it's effectiveness over the years probably becuase it only decreases DHT, rather than inhibit its formation completely. So after some time the remaining DHT still causes loss For some this may be apparent at 5 years, and others maybe 10, or even as little as two. And even then if you get more hair out of it you may very well be as good off as you are today in 10 years. It really depends more on how hard you are hit with male pattern baldness and how well you respond. For many male pattern baldness hits in waves, for some it is a constant thinning towards complete baldness. We are all diferent and no one can say where you will be on the Norwood scale in a year from now regardless if you are on finasteride or not.
If I were you I wouldn't worry about what may or may not happen in the future and just be happy if you can get some slow down in loss or improvement in the present