I have some cast bullets here that are about 22 years old, and the lube in them is still as good as the day I bought them. It hasn't dried out in the least as far as I can tell, and they still shoot exceptionally well in my guns.

I would stay away from the real hard lubes that so many of the commercial casters like to use due to the cost and ease of running it through their sizers. It doesn't lube worth spit as far as I could ever tell, and recovered bullets I have found in my backstop still have it clinging tightly to the lube groove, so obviously it didn't do its job very well while traveling through the bore.