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?
Oh nice one.
You can also add another layer by adding “you know that [infrastructure requirement] from [notoriously uncooperative infrastructure team] is gonna take at least Z days right?”, and if it isn’t enough, go for the kill shot “I’m gonna start writing the architecture principles this sprint, getting back to you as soon as they are cleared by [notoriously picky and detail obsessive software architect].”.
Made me want to become that notorious software architect, just to be the reason that “X is going to take Y days. Want faster? Change your fucking expectations/scope of X”