It doesn't matter what product you use. Everything has a shelf life. A best before date upon which to be used.
Unlike food however, we can actually increase or decrease shelf life. What would you say if I told you some of my resins are close to 2 years old and still good? That they still retain all the properties they should when the cast is fully cured.
Whats Homeworlds BIGGEST piece of advice? STIR WELL. Always stir your products in their original container and then mix well.
Next...Air is the enemy. In all of casting from the mold to the cast to the liquefied products...air is the enemy. I always try to get as good a seal as possible on my products when putting the lids back on as possible. Most times I need a small screwdriver to pry and a pipe wrench to get the lid off from some of the overspill drying. I consider this a good thing. It adds to an airtight seal.
Just because its 6 months old doesn't make it bad. Try it first. Don't just assume its garbage. Make sure its mixed well in its original container. Remember...2 minutes is a long time. Your arm should be tired. Then when you combine the products, be sure to mix well again. Most problems can be attributed to improper mixing. That's why I prefer products with a slightly longer pot life.