Nyeshet said:
I'm rather certain the Worlds Largest Dungeon was crafted, at least in part, with magic. So considerably less time would have been needed. Perhaps months instead of decades.
Let's see... if you were just using magic for construction, and only worried about the overall map of the dungeon (not traps, doors, fine detail, and the like), then a level 25 sorceror with the right spells could build a fairly sizeable dungeon in surprisingly short while, working alone:
Move Earth (to dig a large pit for constructing the dungeon using Wall of Stone): 750x750x10 per casting, 4 hours, 10 minutes per casting. Six of them in a day (well.... 5.76, roughly, in a 24-hour period... but let's just ignore the extra hour, and call this one day) would yield a pit for your dungeon 750 ft wide, 750 ft long, 60 feet deep.
Wall of Stone (to fill the dungeon back in) produces "one 5-ft square a level" of stone (can double the area by halving the thickness). Move Earth has given us a pit that has a "wall space" (4 walls, cieling, and floor) of 750x750x2 (cieling, needs reinforcement option, which doubles square cost) + 750x750 (floor) + 750x60x4 (4 walls, 750x60 each)=750*750*2+750*750+750*60*4=1867500 square feet, or 74,700 5-ft squares. With 50 such a casting, the Sorceror will need 1,494 castings. As Wall of Stone is a 5th level spell, the Sorceror can cast it quite a few times per day - 6 5th, 6 6th, 6 7th, 6 8th, 6 9th, for 30 castings a day (assuming no bonus spells at any of those levels, and assuming that the Sorceror is willing to blow all useable daily slots during construction), giving 49.8 days for the shell. Technically, that's 49.8 resting periods - if the Sorceror camps as soon as the spells are used up, for a total of 9 hours at a stretch (8 hours rest, 1 hour casting), the Sorceror can get nearly 3 (2 2/3's periods) in a single day, reducing "shell time" to roughly 18.675 days.
Note that these are "mage days". 50 such sorcerors, working in concert, could put that shell up in a little over a day - most the time spent moving, dirt, 30 rounds or so per caster for building the stone shell.
Of course, that just gets you a big, honking, underground room, suitable for playing several simultaneous games of football (if you light it up somehow - and ouch, falling would hurt (stone floor)). The actual *dungeon* will take a lot more time, as you are back-filling the room with walls and hallways and more floors and .... and then putting all the dirt back into the spaces you didn't mark off for rooms.