I think fixed lives for different assests are pretty standard regarding similar assets. ie - a computer has a useful life of x number of years in most banks. Although, I'm not sure where that guidance comes from.
Looking at other banks is definitely a good start. The IRS might be another. Your externals might be a good source as well (they help me out, as well as review useful life during their external audit.)
With the lights out, it's less dangerous.