Start taking lessons right away. Your plan starts as soon as you purchase it. However, the week on Cambly starts at 3 am on Mondays.
Let's say you purchase your plan on a Friday, take lessons on Saturday and Sunday. If you don't use all of your time, it will expire at 3 am on Monday. Don't worry, by the end of your plan, you will have at least four full weeks of lessons each month.
EXAMPLE You have a 1.5 hour plan and you have only used 1 hour before 3 am on Monday. The remaining half hour expires and is no longer available.
Friday, September 8-10 = (up to 2 lessons)
September 11-17 = (up to 3 lessons)
September 18-24 = (up to 3 lessons)
September 25-October 1 = (up to 3 lessons)
October 2-7 = (up to 3 lessons)