Not true Wannabe, the SI letter is for 6 years ....usually.... each year certain things must be submitted to the FAA to keep your SI active. The AME will issue the annual medical and the regional flight surgeon will review the data to be sure you are maintaining the standards set forth in the SI. At the end of the 6 years the SI letter that the person must carry will be reissued and can continue to be reissued every 6 years until either the pilot along with his AME petitions to have the SI removed, usually meaning the condition is now under control an no longer requiring the meds needed in the past or the pilot no longer requires a medical thus not getting his SI. YES the state that the medical is not valid for ANY class medical beyond the annual one year point is on the medical and is a give away.

I have a special issuance and I choose to have my AME also be the treating MD for my condition I think it makes it a bit easier when your MD understands what the FAA wants and needs to get the SI.

Bottom line.. is you can have an SI the rest of your career if needed, but the letter must be reissued every 6 years.

disclaimer...different conditions may have different SI letter times.... mine is 6 years.