You didn't say what kind of medication your stepdaughter was taking. I will presume that it's Ritalin®, since that's the medication most commonly prescribed for ADHD. You should be aware that some children take the first dose of their medication in the morning before they go to school so that it takes effect before school starts. This is especially important for children who need the medication to help them control the impulsive behavior that can get them in trouble even before school starts. Some children who do OK on a long bus ride to school may wait to take the medication (from the school nurse) when they get into the building.
Most children take at least two doses of Ritalin® during the day (which means they would get the second dose about midday), unless they take the extended-release form of the medication, which is supposed to last over a longer period of time. Whether or not kids take medication at night or on the weekend depends a lot on how effective it is at those times. Many children who have a hard time focusing on homework take another dose of medication shortly after they get home from school. Most doctors recommend that children also take the medication on the weekends and in the summer, mainly because the symptoms do not go away when school is out. This is especially important if children have lots of homework on the weekends or if they are involved in activities such as sports, scouts, or camps. These are activities that require organized thinking and behavior that is not impulsive. A lot of academic and social gains made during the week can be undone during a weekend filled with disorganization and chaos.