I have an option of buying 5kg of cocoa butter but its more than I need right now. The lady selling it said it had a "use by" date of june 2013, I doubt veru much I can use it all before then. Is it going to expire or something? I have read elsewhere it is pretty indestructible.
I can't speak authoritatively, and I'm sure others here will have better information, but I'll mention my experience. Cocoa butter resists rancidity better than other fats, but not indefinitely. I have kept solid cocoa butter (food grade) that I use for decorating for about three or four years in the freezer and I have not noticed any significant off notes of rancidity. I have even stored Mycryo (powdered cocoa butter) in the freezer with no appreciable rancid notes for 2 years. I don't know if it lasts longer because I end up using it.
Disclaimer: Even though I have had luck with freezing, freezing is tricky. I live in a very low humidity (10% -15%) environment with dew points often below 32 degrees during the winter, so defrosting poses less risk of water condensation for me than in many locales. Because you don't want water to condense during defrosting, it can be tricky freezing in a way that prevents this. If you had powdered cocoa butter that was coated with water and used it with chocolate - say to reduce viscosity - the chocolate could well seize. The risk would be less with solid cocoa butter that you could blot carefully.