All packages have an expiration date. Buying a package implies that you know it and you accept it. If you need more information about your package validity, ask your teacher.
You can schedule your lessons 4 weeks in advance. That means all your lessons must be booked before the expiration date but you can choose to schedule them up to 4 weeks after that date.
10 sessions packages have 70 days validity. That means you have 70 days to schedule your lessons, but when scheduling you can select appointments for the next 4 weeks. For example: your package expiration date is on May the 1st. You can only schedule until May the 1st but you can choose to schedule your appointments until May the 30th. That means you have in reality 100 days to take your lessons ( more than 3 months).
Special packages for less than 10 lessons have shorter validity date. Monthly offers lessons must be scheduled right after scheduling the package (you have only 3 days to do so)
What happens if I haven't booked all my lessons? Before the expiration date, inform your teacher and ask for a 15 days package extension. She'll accept only if she has availability enough.
Some special offers can have very short validity and you can't ask for an extension in those cases.
Why do packages have an expiration date? If you buy a package the price of each lesson is cheaper of course. This is a special offer to thank those of you who take at least weekly lessons. If you want to take lessons once in a while only, then you'll have to buy lessons one buy one. You need to choose a package according to your availability to take lessons: 10 lessons package = 1 lesson or 2 lessons a week minimum
Another reason why there's an expiration date is because buying a package implies a commitment from your teacher: it means the teacher makes sure to have the availability for you and time enough to prepare your lessons. And sometimes that obliges her to stop taking new students.