Apologies if this isn't the right forum for this question...

Does anyone know if there is an expiry date on Office 365 Product Keys that you buy from shops? I ask as it seems like MS like to take the maximum amount ($165/year) of money from you if you buy a product key direct, but some places have Office 365 Home Premium for almost $100/year. 

Therefore if one was to purchase one or two, would they expire prior to having to be used for the following year or two's renewal? (Assuming you can use a packaged product to renew)