I think that line was written for dramatic purposes to indicate that he didn't know how long it would take for the Nazis to show up so he had to be as quick as possible. It doesn't necessarily imply that it didn't take him hours and hours. (But I've no idea what the answer to your question is.)
Though I don't know any more details about the events that took place, there are several ways of speeding up the process. As 23kt gold is quite soft, it would be a quite straightforward process to finely divide it and thereby increase the reaction surface area. Additionally, you could (gently!) heat the solution, though at the cost of a more copious amount of nasty fumes.
