I have a few juices that were over 3 years old when I went through my drawer and after shaking them up really well and giving them the look, smell then if they passed those the taste test I figured they were still good to vape, only a very few passed the first two tests

I don't know about the safe or not safe aspects though since most flavor manufacturers say 6 months to a year and around 2 years for PG/VG, I think a lot depends on how it is stored too.