From what I have seen I would say post WWII would be a place to start looking for galvanized. I inspected one of the WWII training bases in Florida that is still being used and renovated in the 90s. I saw a mix of painted and galvanized boxes there. On the other hand all of the original wiring in the CaDZan mansion in Sarasota (circa late 20s, early 30s) was painted boxes.
My guess is the building boom after WWII was when galvanized boxes really took over.
As an interesting side bar, when I tested the old BX as a ground path in those "temporary" WWII buildings with my Ecos tester they virtually all read <1 ohm under load. I guess those boys did good work.