I've always wanted to play a MEGA-DUNGEON with a DM who wants to run one. I love to making a 2-3 level dungeon when I DM but I don't think I can do a MEGA ones.

Generally yes, the DM doesn't need to answer all things (heoght be revealing some secrets after all, where you can ambush, poison food, whatever). BUT he better is prepared after the questions

Map doesn't make any sense. There is no structure or organization beyond "make a huge block of confusing, pointless rooms". Maps are a part of telling a story. The story this DM is telling consists entirely of incoherent yelling and swearing.

Over half of that map looks absolutely miserable to do combat in, for both the GM and the players. I once had an encounter happen inside a 3x3 grid room as a GM. It was awful. There was no room for anybody to be creative in. It was just a boring slogfest for over an hour

IIRC this dungeon was created specifically for dungeon delving, and the mad mage who created it is, as the name implies, batshit crazy, so this one makes sense.

I know a guy who did this. Use the entire worldgen for the overland map, locations, and factions then would handcraft some dungeons in fortress mode. Sounded like a lot of work but worth it.

How many years does it take to play through a map?

Look up how many years it'll take for iron stars to form and multiply that by six.

That Mad Mage just isn't making sense! It's like a purposeful troll on a group with a persistent completionist mentality. When (if) they catch on the DM can help expedite their routing.

It tells a story of a "Mad Mage" very well

Is this the Godherja under mountain map?

The "Mad Mage", Halaster Blackcloak, is a specific character from the Forgotten Realms setting. "Undermountain" is a special place that he has surprising control over as it has been slowly created by him over the century/centuries, and its layout both isn't permanent and isn't there to serve any given group of people. Monsters and sentients sometimes move in, at their own risk. The map as shown is meant to represent its existence at whatever point in time the adventuring party happens to arrive at. And Halaster keeps a remote eye on anyone of interest who turns up. GMs are encouraged to modify the presented material in whatever way suits their interests.