Several recent studies indicate that there's no reason MMI can't be used as long as needed. I've included this information in my new book on hyperthyroidism that'll be out later this year.  But since this is a common topic on the board, with some doctors suggesting that using MMI for more than 2 years is unsafe, I wanted to share this information now. 

Although the newer studies suggest that MMI can be used indefinitely, today I found an older study from 2005 showing that patients with recurrent hyperthyroidism caused by Graves' disease can be treated effectively and safely for 10 years with methimazole.  None of the patients in the study had any serious adverse effects from long-term MMI use.  Compared to patients who had RAI (61 percent hypothyroid), all subjects on MMI alone were euthyroid at the end of the study.

Source:

Azizi F, Ataie L, Hedayati M, et al., "Very long-term methimazole therapy is effective and safe in patients with hyperthyroidism caused by Graves' disease," European Journal of Endocrinology 2005; 152: 695-701.