Last time I was at Barneys I was testing the legendary Cle De Peau concealer ($70!) and the sales associate tried to claim that it lasts for two years. There was no way I was going to buy that, but it turns out she actually had proof. It's on the label! I never realized this- the little cosmetic jar symbol on your makeup boxes is actually an indicator of how many months it should last. Can you believe it? This eyeliner pen, for instance, is good for six months.
Whether you choose to obey the rules and hold onto products too long (or rifle through them at record speed) is up to you, but they are a good indicator of your level of disgusting consumption.