justme-
You asked "if not curable my understanding is that it will come back at sometime?"
Answer is 'Probably.' I don't know if anyone could tell you with any certainty when or if it will return. The point of the Rituxan maintenance therapy is to stem the cancer's ability to return for longer than the chemo might put it off. That's really what you're doing right now, what might be called preventive immunotherapy (although I just made that term up so don't re-use it and expect anyone to know what it is ...).
As for why only 2 years ... it's possible that studies show that any longer than 2 years does not provide any additional benefit compared to the potential toxicity profile, or that, seriously, someone several years ago, using the drug's general profile, postulated that 2 years was a reasonable time frame and studies ever since have adhered to that 2 year structure. Or maybe there's some really sound science behind it.
"If it comes back is it harder to treat?"
Common sense says yes-- the thinking being that when it returns it does so armed like a virus with the smarts to resist the last treatment-- but common sense also says the earth is flat. There are a number of so-called second-line therapies for follicular lymphoma, and they differ from the first-line therapies (i.e. in terms of drugs used and regimens assigned for combination chemotherapy) but they also include things like radioimmunotherapy -- Zevalin and Bexxar. This kind of addresses your question about whether different drugs will be used-- the answer is probably, although it depends.
All that said, your cancer was graded at 3 so, **** I kinda wish I'd remembered that sooner, but grade 3 FL is aggressive enough that it might be considered curable. If I look up second-line therapies for FL grade 3, I find that I'm directed to the same second-line therapies used for diffuse large B-cell lymphoma. Remember that the primary reason your cancer is not, generally, considered 'curable' is because it grows too slowly to be fully responsive to chemo and radiation. The more aggressive the cancer, the better it 'should' respond to these treatments--- why? Because human cells, healthy and cancerous, undergo a 5-phase cycle of division to grow. Chemo drugs are often cell cycle specific, meaning they target one or more of the phases of the cell cycle. Combination chemo puts chemo drugs together that target different phases so that whatever phase the cancer cells throughout the body might be in when the drugs enter your system, the regimen should have them covered.
So, the more often the cancer cell undergoes division, the more vulnerable it is to one or more of the chemo drugs to step in and stop the cycle, killing the cell.
Since your cancer is grade 3, it's more aggressive than most FLs and therefore potentially more 'curable' than lower-grade FLs.
Hope some of this makes sense,
Ross