No later than the 7th week. And that's almost two months, as we know.
Firstly, we were talking about a major battle, not about clearing out neutral creatures near the entrance/exit of a Dungeon.
Secondly, "no later" doesn't exclude "earlier," quite the opposite.
Thirdly, if you had cleared out those neutral creatures in the 4th week, the final battle would have taken place around the 6th week – that's 7 Cyclops on the opponent's side, not 17 (and the other troops accordingly). You would have even exchanged Furies and Raiders for Cyclops and Centaurs before the main forces clashed. Then, one enhanced Shock attack on the Paladins, and the advantage would already be on your side. But you dragged it out to the 11th or 12th week, and didn't even use the right tactics, not to mention the mistake in choosing the Hero.
Творят настолько черные дела,
Что до таких блистательных идей
Не вдруг дойдет и записной Злодей
Темный адепт клана GODS