Planning for the worse – train to rebuild working systems

It's one thing to get a full infrastructure finally managed by Chef—block by block, weeks after weeks, modification after modification—keeping the Chef run always smooth and working. However, it's something quite different to be able to rebootstrap a working system from scratch. What if the current setup that works perfectly well is in fact working because there's a script or a binary somewhere left from last year, which does the thing that makes it work? What if the application servers get corrupted tonight? If this happens, will we be able to rebuild it from scratch? If tomorrow our IaaS cloud provider crashes, in what timeframe will we be able to rebuild systems somewhere else (provided the backups are working; well, that's another story)?

Now our systems are as much as possible automated, hopefully 100 percent. It's important to know whether we'd be able to fully rebootstrap these systems in case of a disaster; if yes, how long...