28 day if it's unrefrigerated. If you store it properly and don't roll it, you should be able to use the pens to the last drop and the vial anywhere from 4 to 6 months. If you're home-testing, you'll usually see the lost potency reflected in higher BGs when the cat may have been stable at a dose for a while. You also might see cloudiness or particles floating around, which is another sign the insulin may not be good any longer.
In my case, I'd had the vial for about 5 months and I knew it would be going bad soon. (Honestly, for the way I treated it, I was surprised it had lasted that long!) I got a prescription for the pens and got a pen as backup. About a week later, I started noticing what seemed like a higher occurrence of skipped/fur shots (I.e. cycle where it seemed like he had gotten no shot). I switched to the pen for about a week and his cycles stabilized. I tried the vial again and it seemed like another missed shot or a reduced dose, even. That's how I knew the vial had gone bad. With the pen, Mikey is on 2.5u 2x/day currently and I just finished the last pen to the very end (about 2 months' worth or so) without any hiccups.