You might be misunderstanding what "minimum amount of work" means. The OP might simply be referring the minimum amount of work achievable without overexerting oneself via extra hours, working weekends, under stress, etc. Someone's minimum amount of work can be hard work. They literally said "I will always finish my work in the time it is expected to be finished." So if that amount of time is not satisfactory, it is up to the manager to ask for the time to be shortened- and then that becomes the new minimum.