No drugs come with a guarantee that they will not stop working after a while. Why not try and see if it works for you. Surely you would rather have the chance of an improved quality of life, even if it's not permanent?
It really varies from person to person. Some have been on it for a long time, some might only have it last several months, some may never have it work for them. If your doctor recommends it and your quality of life is still not all that great, then go ahead and try it. It is worth a shot isnt it?
I was on Humira for about 18 months before it stopped working for me. It took several months to start working and then my doctor put me on quite a high dosage of it (80ml/2 weeks). It gave me a year of solid relief. I know it sucks, and no one likes to be jabbed every other week but you should definitely try it. My doctor wouldn't move on to trying anything else until I had given it a try.