Everyone is going to be different, but I've always had a problem with meds becoming less effective after about a year. For about 10 years, I've had to switch back and forth on acid reflux meds because they only seem to work for about a year and then stop. I was on Remicade and lost response and had to stop it (due to allergic reactions) after about a year (starting losing response before a year...maybe 6 months in), Humira seemed to lose effectiveness after less than a year and I just switched back to Cimzia.
I don't know the technical (or physical) reasons for it, but I think your body just gets used to it over time and so it doesn't work as well. My husband has a bad back and has the same problem with pain meds....after being on one for 6 months or so, it stops helping, so he switches to something else. He can switch back to the one that stopped helping and it will work again.
Crohn's Disease, Acid Reflux/Gastritis, Hashimoto's Thyroiditis, Endometriosis, Arthritis, Depression/Anxiety. Too many meds to list them all. =)