How close to build a redundant data center

This is a disaster recovery question. I’d like to know if there are any industry rules of thumb to advise us on the distance at which we should place a permanent local redundant data center. We want it to be close for staff convenience, but how close is too close? How many miles would “typically” be evacuated for a “typical” environmental hazard, for instance? Does anyone know of any good resources to consult on this topic?