Manager: We (meaning you) need to do task A. How long will it take?
Me: Task A will take X days to do.
Manager: That seems awful long.
Me: How long do you think it should take?
Manager: It surely could not take any longer than Y days.
Me: Ok, it seems you have an answer to your question then.
Later:
Manager: It’s been Y days, why isn’t task A done yet?
I used to hate this. I still hate it, but I used to hate it, too.
More seriously, something that has helped from my managers is getting a “product requirements doc” or PRD. Management and customer reps put together a requirements document that says what they need built. Then you can break it down into work units for yourself and estimate the size with milestones and deliverables. It helps a bunch, even if it’s extra overhead at the start.