The useful life of an appraisal is not a defined time. Each situation is different. Some things to consider: 1) is this just a renewal that will only change rate/pmt amount, 2) any new monies advanced, 3) any credit deterioration. I would presume from your post your Board Policy is not clear about this; so it is a fair question to ask here.
I would recommend you look at the Final Interagency Appraisal & Eval Guidelines, Dec 2010. This has a section for Transactions that Require Appraisals; it includes discussion on the 12 exemptions. Other guidelines are out there, but I find this one easy to reference and renewals/refi's are discussed.
If you conclude a new appraisal is needed, we are seeing the use of "updates" from the original appraiser. This is actually a new report, but with more limited scope. It is useful and a good tool to get where you need at the end of the day: to monitor your collateral.