Sustained attention time is only partially determined by personal choice. Physical and mental strength probably control a larger percentage than most people would like to believe.
We know the answer to the question about why sustaining attention longer is important. When people have nothing to do all day after self-care is done, they get into trouble. When no one knows how to keep them busy, nurses ask doctors to prescribe antipsychotic medications. Antipsychotic medications keep them out of trouble by further reducing their ability to function, probably to ACL 1.6 or 1.8, with associated costs and consequences, i.e. loss of ability to walk.
Within ACL 3.2, the mental power to sustain attention is probably less than 15 minutes, and then a rest period will be required. 2 sessions a day, scheduled for 45 minutes might be enough to prevent troublesome behaviors. Because eating is so slow, an hour a day may be required to feed themselves. 2½ hours a day of something to do may not be enough to keep them out of trouble.