To answer your rhetorical question Yes that s about right HMS Valiant 1759 which is featured in the Chatham Historical Dockyard Wooden Walls exhibition took 26 months from ordering to launch While HMS Montagu 1779 took a month from launch to sea ready Chatham was quick when it needed to be I suspect that a repeat order that could use the same moulds could shave a couple of months off of that time making it posssible to have a sea ready vessel in under twenty four months