This is what my hero looks like.

Yeah, guys, you're real Might and Magic 8 experts. I played it a long time ago, but if I remember correctly, there's an elf you can recruit around day 7-8 who has a full arsenal for victory: a portal town, invisibility, and that's all you need. By the way, Might and Magic 8 doesn't really have much of a plot; there are very few story quests, and they are completed very easily with invisibility. I'm telling you, at the end of the game, his first hero was only level 3, and he didn't kill or level up anyone! And then I said, I think, maybe it was 15-20 days, but definitely less than a month. This was about 2 years ago.

Kauri Black Sail joins at a minimum of 12 days: 5 days before the Battle of the Bridges, 2 days before Alvara, and another 5 days before Shadowlands. And she doesn't have any "Master" level spells. Also, the third level of the hero, by the way, requires 14 days of training. That's already 27 days. Plus, you have to travel to Wild Pastures (to form an alliance with the minotaurs), which takes another 15 days (5 days from Shadowlands to Underworld and 10 days from Underworld to Wild Pastures). And then to Peak of Shadows, to Iron Sands, to Regna, and further. It's impossible to do it in less than a month.
By the way, level three isn't a benchmark; I know a walkthrough using a level one cleric, but even then, it took more than a month to complete.