Good question, and one for which I wish I had an answer.
A very knowledgeable person has said, in a nutshell, that liquid T3 has a very short shelf life (the issue isn't stability in solution but decomposition). However, when I ran that by Patrick Arnold, he more or less disagreed.
If I were you, I'd keep dosing upward until you experience symptoms associated with Cytomel, if any. If you're up to huge doses with no effect, then draw your own conclusions. However, just because your T3 ends up bunk doesn't necessarily mean al;l liquid T3 is bad, either.
Bottom line: it's a crapshoot. When I add T3 to my cut in March, I will have 2-3 bottles, from various sources, in case the first doesn't work.