Little to no revenue would be lost as I have built into my model a two card failure system. I have two cards that I tested that are sitting on a shelf as spare parts. I would replace the broken card with that one, then order a replacement with either overnight shipping or second day shipping as I have free two day shipping with Newegg and Amazon.
Wait so instead of losing 2 weeks of revenue when a card does die (if/when that happens) you will lose 50 weeks of revenue the hot spares could be generating all the weeks that they aren't running?
Well whatever works for you.
Right now the quantities of cards I am ordering frequently puts Newegg out of stock on the models I order. I picked up the spares to serve as fill-in cards when I get bad cards right from Newegg as well as hot spares for what I would consider a premature failure. Once I stop expanding the operation, and quantities are more readily available, then I would probably not have a hot spare as a direct replacement is only 48 hours from my door step.
You just asked me how much revenue I would loose, and based on my current growth and operation the answer is none.
Once my operation is done expanding the answer to your question is I would loose 48 hours of revenue for one single card at most.
Edit: Eventually many cards would be replaced with BFL singles once my orders start shipping, but I didn't want to put my entire investment in one basket with BFL and wait a couple months not making any revenue. I wanted to start and get a solid 8 months of mining in before the inevitable reduction in reward.