A survey done a few years ago with a Year 9 class revealed some interesting things. One was that almost no one in the class had a particular space set up for doing homework in the evenings or on weekends.
One boy, who lived on a narrow barge with his parents, used a foldable table which he had to clear and lift up whenever anyone else in the boat wanted to move from one end to the other! The most usual situation was that children were expected to be doing homework anywhere in the house -the kitchen table, a corner of the living room, a shared bedroom -and in all cases the spaces were open to traffic and interruptions. This was non-optimum to say the least: the battle to get homework done was being made much less easy to win. A child normally needs a dedicated, quiet space in which to work if he or she has any hope of completing homework assignments of good quality.
Similarly to needing a stable space in which to work, when asked 'At what time do you usually do homework?' answers varied hugely from 'during school if I can' to 'after dinner' to 'in the morning before breakfast' to 'on the bus', the most alarming pattern being that there was no pattern -children just did it as and when they could. Establishing a clear time for homework, as part of a comfortable routine, is the way for children to become more successful at learning outside school hours. Imagine if you had to do your job, but had no plan as to when or where you would do it -and the time changed everyday! Result? Not much work done.
Along with having a dedicated space and a clear routine, the other thing to watch is sleep. Many studies have been done on this. Most confirm what common sense would say: get enough sleep, and plan to get more than enough. A rested person is capable of tremendous things; a tired person has lost before they start. Teenagers in particular need lots of sleep and tend to get moody and miserable when they don't get it. Lack of sleep is a primary enemy on the road to learning success. Encourage plentiful sleep as a normal thing.
Sitting at a desk, reading, writing or calculating can seem like a relaxing proposition to anyone engaged in hard, physical labour -but the energy burned up in taking in data or getting involved in creative activity can be comparable to that expended in manual work. Hours of school, being shunted from demanding subject to demanding subject, can be just as physically exhausting as a marathon run. This should be allowed for in working with children, especially teenagers who are also being introduced to more challenging specialist subjects. Make sure they're eating good food in sufficient quantities to keep them 'fuelled up'! Think of every hour at school or doing homework in terms of calories burned and you won't go far astray.
As it's best to have an established routine for homework, so it's even better to have an overall stable and predictable routine in a child's or teenager's life. Some variation to avoid boredom is good -but try to give plenty of warning of any changes so that everyone has time to plan and adjust. It's always a good idea to consult a teenager too -they are busy making plans of their own, remember!
Setting up a space, a routine, and keeping basics in is a recipe for a healthier and more productive school career, especially when things like homework are concerned.