I do the 'sniff' test. If it starts drying out, smells rancid, or starts turning a funky color then it is definitely time to toss. I don't do the calendar test since I have had my Prep + Prime more than 3 months and it still works fine. I know it is against the cosmetics company 'rules' however I think some things last longer than the 'recommended' time frames if you store it in a cool, dry place.
Oh, also wanted to add that I also think the amount of time that you use it should also be in the equation. If you have a product for 3 months and only use it a couple of times (thus exposing the product to the elements and to your lashes,skin), then I wouldn't toss it just yet. If you use it pretty much daily for three months straight, then you're looking at getting a new one when the time is up since you are in contact with it for a much longer period of time. JMHO. HTH!!!