Occasions where it seems you have no option but to fix someone's personality come up surprisingly often in language teaching, but that doesn't make it any easier to do. Options for this problem include doing a lesson on how they use their time and time management, finding out when they do have time and designing the homework to fit in with it (e.g. a recording they can do in their car or a compact self-study book such as a graded reader they can do standing up on the train), telling them how long each exercise should take, or even asking them to write the time and date when they are going to do it rather than the date it must be done by in their workbooks.