Remember that manager who asked:
— Hey Adrian, we have this thing the client wants. How long do you think it would take?
— I don't know. I'd have to think about it.
— Off the top of your head. Give me a number. We'll discuss it later.
— I think it would take 4 days, max.
You know that's the end of the discussion. Your only "later" discussion will be about the 2 weeks you spent on that task and how much longer do you think it would take. You were right to "think about it". Always "think about it". Knee-jerk reactions are not valuable, they are actually detrimental to your work from that oint on.