Analyze your schedule to determine why you fell behind in the first place. You will need to work around the circumstances that caused you to fall behind. Sometimes, it may be obvious that you or your children are over-committed. It may be less clear if you are falling behind one lesson at a time because your child is getting bogged down in a particular subject. Be ruthless about giving up extra commitments that are keeping you from getting caught up; you can always pick them up again at a later date.
Study your state's homeschooling requirements. Each state has different curriculum requirements, and there may be more specific regulations for high-school students than for elementary students. Either way, compare state regulations to your current curriculum. If you are trying to accomplish way more than your state requires, you may want to set aside any elective or accelerated classes to allow you more time to focus and get caught up on the required courses.
Look at the calendar. If you are seriously behind, this may mean working into the summer; if you are in the depths of winter, you may find it worthwhile to add extra schoolwork to your child's day to catch up by the time nice weather arrives. Your time constraints should be based on what is convenient and beneficial for your family and your situation.
Make a schedule with serious deadlines, once you have pared down your course list. This can be as simple as dividing the number of lessons remaining in a textbook you are behind in by the number of days you have left until you wish to be caught up. You may wish to focus on one lagging subject at a time. Depending on what works for you, you may wish to create a schedule based on lessons-per-week, or one that maps out what your child should be working on each hour of the school day. Once you make your schedule, review it to make sure it is realistic, and commit to working day by day to complete your tasks.