Most people say a year but I have used flavors that were much older and they were still fine. As most flavors are made for use with food as well they will last a long time. I've had bottles of liquid flavors in my kitchen cupboard for well over a year and they were fine.

Flavour Art's "best by" date is about 1.5-2 years after the manufacturing date, if I am decrypting their batch numbers correctly. I'd imagine other brands are similar. The trick is, if you're buying from a reseller, you don't know when they got it and rebottled so I'd safely say a year.

I've heard 2 years. But not 100% sure.

I've got flavourings that are about 18 months old, they're starting to get a little tired.

2 years from date of manufacture, longer with loss of flavor.

Only flavor that went really bad was blue raspberry cotton candy by Capella after about a year and a half .

Recently found 4ml of TFA Cotton Candy(EM) That sat in a drawer for 6 months. Went really bad on me.

Most of the DIY liquids dislike oxygen, heat and light. Depends how you treat them.