You may be correct but I've seen this work out before. You say you take x days, manager mentions " but that guy says he takes y days!!" in which makes you either slow in comparison or the other guy is a liar or makes stuff sloppy. Then you say " from my experience is x days" in which either the manager says you're wrong or he goes to the other guy to do in y days, and when you look at the thing done in y days has a technical debt equivalent to the height of Niagara falls. And you know you will have to clean this later. At some point I walked away, sometimes managers just want a yes man, and I am straight up not that.