DNS cache hold of SERVFAIL responses

This is for any Google admin on this list: When you receive a SERVFAIL from a name server listed as authoritative for a given domain, how long is that negative look-up cached? When you receive a SERVFAIL from the root servers, how long is that negative lookup cached? Does Google follow RFC 2308? Is there a common cache for all resolvers, or do each resolver in your DNS server corps maintain a local cache? When a SOA (when you see one) says 14400 for the minimum TTL (or negative cache TTL) does Google honor that hold time?