I think "We work the whole month" is probably better English.
Works as well.
"We work all month long" has been rejected. Is this an incorrect translation?
Sounds fine, it's just idiomatic rather than literal. Added now.
Why is "We've been working for the entire month" incorrect?
Oversight, added now.
But "we've been..." is past tense
Well, that's Present Perfect. It's not exactly Past Tense.
Why is this Nominative?
It's not, it's in accusative. It doesn't change because 'miesiąc' is inanimated, masculine
Masculine and inanimate, to be precise. Feminine nouns change, no matter whether they're animate or not.
Yeah ok, added. Thanks!
What is the rule here for this to be accusative? It is not an object of the action.
Time 'for how long'? That's how it would be in Latin: laboramus totum mensem.