I think the lifetime is something like 3-5 years, at least for Thuricide concentrate, if stored in a cool, dry place. I've seen 2 years quoted in some places, and at least one person saying that 10 year-old Thuricide worked fine for them. It is curious that the labels themselves don't admit any expiration period. I've spent some effort trying to figure out if you can freeze the stuff and have it thaw in a usable form to extend the expiration period. I can't get a straight answer out of the sellers or the manufacturer.

I guess the answer is that it works until it doesn't work.

Assuming that you are talking about liquid BTK like the Bonide or Safer Brand, yes it has a limited shelf life. This link from Missouri Botanical Gardens says it's best to use it within 2 years of purchase. BTK dust has a longer shelf life.

I would try it and see if you see signs of it working indicated by caterpillar carcasses. I usually use up most of a 16 oz. bottle a year and have never used any older than 1 1/2 years myself.

Much depends on storage conditions. I keep mine indoors, on a high shelf in the laundry room, and have been surprised that it still works after 3 years.

It has been sitting in the garage so I'm not sure how effective it is after 2 years. I'll have to try and see.

Last year I didn't bother with BT and used tulle to cover my kale/chard beds all season. This year I'm also growing Brussels sprouts in the same bed so i figure treating with BT every 10 days or so should help ensure a successful BS harvest.