There are a lot of variables that will affect the longevity of your flavors... mainly the type of volatiles that make up the flavor (and the concentration of those volatiles), then the type of bottle you store them in, along with the average ambient temperature of the area they're stored.
Most flavors don't go bad (develop off notes) but just get weaker as they lose potency and because different volatiles escape/degrade at different rates the overall profile of a flavor can sometimes change.
I talked a little about it in this post and linked TFA's response on shelf life.
Depends on a lot of things. The shelf life usually cited by the manufacturers of those flavours is 2 years. You flavours should have an expiry date printed on the package, if they don't - DO NOT BUY THEM.
Go with that expiry date in general (although just as with supermarket products, they might outlive the expiration date). When flavours go bad, some people reporting getting bad/awful taste of the flavour (since the chemicals inside have started changing). How that affects your health - I am not sure.
The problem with not buying them if they dont have an expiry date is kinda hard when you mail order the flavors. Also i dont think i ever seen that on any TFA , Capellas, flavor west flavors.
No, it isn't, I do exclusively mail order as well. Just buy off from the reputable brands that are cited around the subreddit. One comes without a label, is shady looking - toss it away and don't order again. Since these flavourings are usually used in foods, they are required to have an expiry date in most countries they are sold.
See this image of a FlavourArt flavouring I have bought locally As you can see, the end date has even been translated since it's a requirement in the EU to translate the label with important information when importing for sale.
Honestly there’s sooo much more to learn and worry about w this hobby, trust me that’s the last thing you should worry about lol...or that’s what I keep telling myself as mine keep getting older and I buy more instead of using what I have. You know expiration on food is really waaay before it’s technically bad just to have the worse case scenario dates for a rare occurrence. I feel the expiration dates on our flavors probably much the same and hopefully last a whole lot longer if stored properly and air conditioned. I have many 8oz ones I got cheap on a close out that I’ve worried about the same way. I seriously doubt I’ll get to them, but I don’t get rid of them bcuz WHAT IF I do need them 🙄 and don’t have them LOOL.. it’s sick 😩😂! Hope that helps...;))