From my experience, you can keep a starter forever, essentially, as long as you feed and maintain it properly, and as long as it doesn't show signs of going bad (unusual smell, color, etc.). (brighteyedbaker.com)
Levoxyl is on long term manufacturer back order until mid-year in 2014... An unusual smell noticed by patients and pharmacists who opened medication bottles prompted the manufacturer to recall all strengths of Levoxyl. (thyroidpharmacist.com)