Which faction do you like the most in Heroes of Might and Magic IV?

First, you need to level him up to that point. Once you've leveled him up and, in addition to Stealth, also leveled up Combat to level 5, he definitely won't be a problem for me anymore; the army will already be too large... Besides, I was going to attack you in the first place, and that will happen long before you max out Stealth, let alone Combat.

Shall we consider this?))):)

Stealth is leveled up at roughly the same time as Necromancy, level-wise.
But the Rogue gains 75% more experience if, after running around a monster, he also attacks it (the experience will contribute to Stealth and then to Combat). It's not hard to figure out how quickly a Rogue levels up; plus, the Rogue's movement speed will be quite good.

Too large an army??? When? Later than the 3rd week, or earlier?

I've already explained how to level up the Rogue. Basically, if the Rogue has the opportunity to gain experience from Stealth on the map, he will level up faster than any other hero.
So, statements like

the army will already be too large... Besides, I was going to attack you in the first place, and that will happen long before you max out Stealth

are unfounded:)