I would love it of flavoring lasted 2 years. However, other sources I've heard from (E.g. TFA) say that shelf life is between 6 and 12 months. Others say that it depends on the individual flavoring, someone saying that Hangsen tobaccos will last 2 years. Finally, I've heard that the nature of the container is also a factor as well as how often the container is opened.
I've had most of my flavorings for about 6-8 months now and I'll be curious to see how they fare after another 6 months or so. I expect they'll slowly fade out and I'll need to up the percentage of some/most/all my recipes over time. I'm also budgeting for the prospect of replacing them within a year or so, hopefully not much sooner than that. When I do replace, I think I'll split off half or so of each flavor into tightly capped glass vials and perhaps stick those into the fridge. (I've also heard that freezing can 'damage' some flavorings.) While the dropper bottles are convenient, I have to believe they let the flavoring out. Before I started keeping them in tight wooden boxes, my office would really reek of flavoring. That must mean that the essence of my flavorings ($$$) are fading away, drop by drop. Into the air means out of the bottle.
Sorry for all the "I think"s and "I heard"s, but those anecdotes and common
sense are all I have to go on.