When my manager says you can do this whole project in 14 days its very easy,

meanwhile project and me

Never met a manager that new what they’re on about

The correct answer is "Sure, I will assign 14 days to this and see how far I reach"

Potentially adding: "If I'm not done by day 14 do you want me to stop working on it? No? Well in that case why not just say that I should work on it 'until it's done' and I'll post some updates along the way"