Remind parents that they can help make study time more productive by creating a quiet atmosphere. Also, while it's important to keep to a regular dinner schedule, parents should be open to shifting the dinner hour slightly to accommodate a special homework assignment or study session before a test. By doing this, parents send kids the message that schoolwork is a priority. Build in a breather. In fact, breaks can increase students' productivity.
Attention spans vary from child to child, often increasing as the child gets older.
Parents know better than anyone how long their child is capable of sitting at one time. Help sharpen kids' focus. Often children have trouble maintaining their attention because they underestimate how long their assignments will take to complete. One teacher I know helps her children be more realistic about time by working with them to approximate how long an assignment will take. For example, she points out that if one math word problem takes three minutes to solve, then five similar problems will take about 15 minutes.