The closest I've come is 37 days. Basically here is how I've done it. Note that there is a fair amount of luck in it too, hoping your heroes don't blindly decided to go dragon hunting too early on. At the start of the level you'll notice you don't get attacked much, this is the time to rake in the cash you need to get your Library up to level 2 and acquire the Meteor Storm spell which is critical (it can kill multiple dragons at once). So...
#1 Build a Market, maybe two (start leveling it up, and don't stop)
#2 Build an Elven Bungalow ASAP (doubles your cash & Elves have good long range attacks)
#3 Build a Ranger Guild & Temple of Krolm (and get some heroes out)
#4 Build a Wizard Guild (don't train any wizards yet, they die too easily)
#5 Build a Library (level it up as quickly as you can)
#6 Train some Wizards as soon as you acquire the Meteor Storm spell
#7 Build Guard Towers or if you can a Second Wizard Guild
#8 If you're really in the money and have enough men, upgrade your Castle to level 3.
As always, you may have to babysit the Wizards and cast Invisibility on them to make sure they stay alive. Barbarians are cheap and quick to train and will keep the undead in check. Rangers help backup the Barbarians with ranged attacks. But as a whole expect to lose a lot of these grunts once the Dragons show up (I've had 6 attacking me at once before), so keep building them as they die and focus on keeping the wizards alive.
Good Luck!
