It seems that two months is the standard amount of time it takes to build any one area of the Refit, as that's the amount of time it took for the Transporter Room, Sickbay, and now the Photon Torpedo Bay as seen in The Wrath of Khan.
Most Trekkies know that the torpedo bay was, in fact, a redress of the Klingon Battle Crusier Amar bridge from The Motion Picture. This is evident in the odd polygonal shapes that are present on some support beams, as well as the deck bracing itself. Walls reused from other The Motion Picture sets make of the aft end of the deck to try and give it a more Starfleet feel.
It is a wonderful set, IMO, and is probably, besides the Refit Enterprise bridge, one of the most complex areas I've ever tackled. Also, this room serves a very important role in Star Trek history, as it is the setting of Spock's funeral and Kirk's great eulogy at the end of The Wrath of Khan.