As most flavors use PG or alcohol as a carrier, my understanding is that the anti-bacterial nature of those carriers keep the flavors sterile, as long as you're reasonably conscientious and not, say, eating raw meat and mixing juice at the same table.

Flavors with decrease in intensity over time, though (as the molecules break down, etc.) which may require some % upping if you're following a tried and true recipe. I've noticed a slight strength decrease with flavors I've had for over a year, but I'd say it takes about 2 years before it becomes really noticeable. In my opinion.

Main things you mix with a lot go with 4oz bottles. We are getting ready to start going with 16oz bottles on some of our more used flavors soon but start with 8ml bottles from wizard labs till you know what flavoring you tend to use over and over.

The only one I know of that goes bad in a shortish amount of time is Sweet Cream by TFA. I believe they give it a 6 month shelf life from the time you get it.

I noticed some of my Capella bottles had expiry dates on them last week (they would have been a year expired when I got them last year), after mixing up 2 bakery flavours that tasted a lot more stale and "off" than they had in earlier mixes with the same recipe. I checked their website and it says "Our unsweetened flavor drops have a shelf life of approx 9-12 months". I haven't had a problem with any of my other flavours from anyone else.

I know some people refrigerate/freeze their nicotine concentrates, could that apply to the flavorings too and just take them out when needed?