To answer your rhetorical question. Yes thats 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.